Polynucleotide modification methods

ABSTRACT

The invention relates to a method for modifying a template double stranded polynucleotide, especially for characterisation using nanopore sequencing. The method produces from the template a plurality of modified double stranded polynucleotides. These modified polynucleotides can then be characterised.

RELATED APPLICATIONS

This application is a Continuation of U.S. application Ser. No. 14/911,853, filed Feb. 12, 2016; which is a national stage filing under 35 U.S.C. § 371 of PCT/GB2014/052505, filed Aug. 14, 2014; which claims foreign priority benefits under 35 U.S.C. § 365(b) of British application number 1314695.6, filed Aug. 16, 2013. The entire contents of these applications are incorporated herein by reference in their entirety.

FIELD OF THE INVENTION

The invention relates to a method for modifying a template double stranded polynucleotide, especially for characterisation using nanopore sequencing. The method produces from the template a plurality of modified double stranded polynucleotides. These modified polynucleotides can then be characterised.

BACKGROUND OF THE INVENTION

There are many commercial situations which require the preparation of a nucleic acid library. This is frequently achieved using a transposase. Depending on the transposase which is used to prepare the library it may be necessary to repair the transposition events in vitro before the library can be used, for example in sequencing.

There is currently a need for rapid and cheap polynucleotide (e.g. DNA or RNA) sequencing and identification technologies across a wide range of applications. Existing technologies are slow and expensive mainly because they rely on amplification techniques to produce large volumes of polynucleotide and require a high quantity of specialist fluorescent chemicals for signal detection.

Transmembrane pores (nanopores) have great potential as direct, electrical biosensors for polymers and a variety of small molecules. In particular, recent focus has been given to nanopores as a potential DNA sequencing technology.

When a potential is applied across a nanopore, there is a change in the current flow when an analyte, such as a nucleotide, resides transiently in the barrel for a certain period of time. Nanopore detection of the nucleotide gives a current change of known signature and duration. In the strand sequencing method, a single polynucleotide strand is passed through the pore and the identity of the nucleotides are derived. Strand sequencing can involve the use of a polynucleotide binding protein to control the movement of the polynucleotide through the pore.

SUMMARY OF THE INVENTION

The inventors have surprisingly demonstrated that it is possible to modify a template double stranded polynucleotide to produce a plurality of shorter, modified double stranded polynucleotides. The modified double stranded polynucleotides may include, for instance, a hairpin loop or a single stranded leader sequence. These modifications can be designed such that the modified double stranded polynucleotides are each easier to characterise, such as by strand sequencing, than the original template polynucleotide. Subsequent characterisation of the modified polynucleotides allows the character of the template polynucleotide to be more easily determined.

In one embodiment, the modification method uses a MuA transposase and a population of MuA substrates each comprising an overhang of universal nucleotides. Universal nucleotides are nucleotides which will hybridise to some degree to all of the nucleotides in the template polynucleotide. The MuA transposase is capable of fragmenting the template polynucleotide and producing fragments with overhangs at both ends. The MuA transposase is also capable of ligating a substrate to the overhang at one or both ends of the double stranded fragments. The strand of the substrate without an overhang is typically ligated to the strand of the fragment with an overhang. This leaves a single stranded gap in the resulting double stranded construct. Surprisingly, the overhang in the substrate containing the universal nucleotides is capable of hybridizing to the overhang in the strand of the fragment (to which the opposite strand in the substrate without the overhang is ligated) and closing the gap left by the action of the transposase (see FIG. 1). Ligating the overhang to the adjacent strand in the fragment produces a modified double stranded polynucleotide.

Accordingly, the invention provides a method for modifying a template double stranded polynucleotide, comprising:

(a) contacting the template polynucleotide with a MuA transposase and a population of double stranded MuA substrates each comprising at least one overhang of universal nucleotides such that the transposase fragments the template polynucleotide and ligates a substrate to one or both ends of the double stranded fragments and thereby producing a plurality of fragment/substrate constructs; and

(b) ligating the overhangs to the fragments in the constructs and thereby producing a plurality of modified double stranded polynucleotides. In an alternative embodiment, the modification method uses a MuA transposase and a population of MuA substrates each comprising at least one overhang and at least one nucleotide in the same strand as the overhang which comprises a nucleoside that is not present in the template polynucleotide. The MuA transposase is capable of fragmenting the template polynucleotide. It is also capable of ligating a substrate to one or both ends of the double stranded fragments. The overhangs are then specifically removed from the ligated constructs using the at least one nucleotide which comprises a nucleoside that is not present in the template polynucleotide. This leaves gaps in the double stranded fragments and repairing these gaps provides a plurality of modified double stranded polynucleotides.

Accordingly, the invention also provides a method for modifying a template double stranded polynucleotide, comprising:

(a) contacting the template polynucleotide with a MuA transposase and a population of double stranded MuA substrates each comprising (i) at least one overhang and (ii) at least one nucleotide in the same strand as the at least one overhang which comprises a nucleoside that is not present in the template polynucleotide such that the transposase fragments the template polynucleotide and ligates a substrate to one or both ends of the double stranded fragments and thereby producing a plurality of fragment/substrate constructs;

(b) removing the overhangs from the constructs by selectively removing the at least one nucleotide and thereby producing a plurality of double stranded constructs comprising single stranded gaps; and

(c) repairing the single stranded gaps in the constructs and thereby producing a plurality of modified double stranded polynucleotides.

The invention also provides:

-   -   a plurality of polynucleotides modified using a method of the         invention;     -   a population of double stranded polynucleotide substrates for         modifying a template polynucleotide, wherein the substrates are         as defined above;     -   a method of characterising at least one polynucleotide modified         using a method of the invention, comprising a) contacting the         modified polynucleotide with a transmembrane pore such that at         least one strand of the polynucleotide moves through the pore         and b) taking one or more measurements as the at least one         strand moves with respect to the pore wherein the measurements         are indicative of one or more characteristics of the at least         one strand and thereby characterising the modified         polynucleotide;     -   a method of characterising a template polynucleotide,         comprising a) modifying the template polynucleotide using a         method of the invention to produce a plurality of modified         polynucleotides, b) contacting each modified polynucleotide with         a transmembrane pore such that at least one strand of each         polynucleotide moves through the pore and c) taking one or more         measurements as the at least one strand of each polynucleotide         moves with respect to the pore wherein the measurements are         indicative of one or more characteristics of the at least one         strand of each polynucleotide and thereby characterising the         template polynucleotide; and     -   a kit for modifying a template polynucleotide comprising (a) a         population of substrates as defined above and (b) a MuA         transposase.

DESCRIPTION OF THE FIGURES

FIG. 1 shows a method of modifying a template double-stranded polynucleotide (a) by contacting the strand with a MuA transposase (b) and a population of double-stranded MuA substrates (c). The double stranded MuA substrates each contain a 5′phosphate (labelled as a circle) and five universal nucleotides (labelled as a rectangle). The MuA transposase fragments the template double-stranded polynucleotide and inserts the MuA substrates at each side of the point of fragmentation (Step 1). The nick which is left in the fragmented double-stranded construct is then repaired using a DNA ligase (d), which ligates the strand containing the universal nucleotides to the double stranded construct (Step 2).

FIG. 2 shows a method of modifying a template double-stranded polynucleotide (a) by contacting the strand with a MuA transposase (b) and a population of double-stranded hairpin MuA substrates (c). The hairpin MuA substrates each contain a 5′phosphate (labelled as a circle) and five universal nucleotides (labelled as a rectangle). The MuA transposase fragments the template double-stranded polynucleotide and inserts the MuA substrates at each side of the point of fragmentation (Step 1). The nick which is left in the fragmented double-stranded construct is then repaired using a DNA ligase (d), which ligates the strand containing the universal nucleotides to the double stranded construct (Step 2).

FIG. 3 shows a method of modifying a template double-stranded polynucleotide (a) by contacting the strand with a MuA transposase (b) and a population of Y-shaped MuA substrates (c). The Y-shaped MuA substrates each contain a 5′phosphate (labelled as a circle) and five universal nucleotides (labelled as a rectangle). The MuA transposase fragments the template double-stranded polynucleotide and inserts the Y-shaped MuA substrate at each side of the point of fragmentation (Step 1). The nick which is left in the fragmented double-stranded construct is then repaired using a DNA ligase (d), which ligates the strand containing the universal nucleotides to the double stranded construct (Step 2).

FIG. 4 shows a method of modifying a template double-stranded polynucleotide (a) by contacting the strand with a MuA transposase (b) and a population of double-stranded hairpin MuA substrates (d) and Y-shaped MuA substrates (c). Both types of MuA substrate each contain a 5′phosphate (labelled as a circle) and five universal nucleotides (labelled as a rectangle). The MuA transposase fragments the template double-stranded polynucleotide and inserts the MuA substrates at each side of the point of fragmentation (Step 1). The nick which is left in the fragmented double-stranded construct is then repaired using a DNA ligase (e), which ligates the strand containing the universal nucleotides to the double stranded construct (Step 2).

FIG. 5 shows a method of modifying a template double-stranded polynucleotide (a) by contacting the strand with a MuA transposase (b) and a population of double-stranded hairpin MuA substrates (d) and Y-shaped MuA substrates (c). Both types of MuA substrate each contain a 5′phosphate (labelled as a circle) and five universal nucleotides (labelled as a rectangle). The MuA transposase fragments the template double-stranded polynucleotide and inserts the MuA substrates at each side of the point of fragmentation (Step 1). The nicks which are left in the fragmented double-stranded construct are then repaired using a DNA ligase (e), which ligates the strand containing the universal nucleotides to the double stranded construct (Step 2).

FIG. 6 shows a method of modifying a template double-stranded polynucleotide (a) by contacting the strand with a MuA transposase (b) and a population of double-stranded hairpin MuA substrates (d) and Y-shaped MuA substrates (c). The Y-shaped MuA substrate has an additional polyT leader sequence (show as a dashed wiggly line). Both types of MuA substrate each contain a 5′phosphate (labelled as a circle) and five universal nucleotides (labelled as a rectangle). The MuA transposase fragments the template double-stranded polynucleotide and inserts the MuA substrates at each side of the point of fragmentation (Step 1). The nicks which are left in the fragmented double-stranded construct are then repaired using a DNA ligase (e), which ligates the strand containing the universal nucleotides to the double stranded construct (Step 2).

FIG. 7 shows a method of modifying a template double-stranded polynucleotide (a) by contacting the strand with a MuA transposase (b) and a population of double-stranded hairpin MuA substrates (c). The hairpin MuA substrates each contain a 5′phosphate (labelled as a circle) and five universal nucleotides (labelled as a rectangle). The MuA transposase fragments the template double-stranded polynucleotide and inserts the MuA substrates at each side of the point of fragmentation (Step 1). The nicks which are left in the fragmented double-stranded construct are then repaired using a DNA ligase (d), which ligates the strand containing the universal nucleotides to the double stranded construct (Step 2).

FIG. 8 shows a method of modifying a template double-stranded polynucleotide (a) by contacting the strand with a MuA transposase (b) and a population of double-stranded MuA substrates (c). The double stranded MuA substrate contains a single nucleoside that is not present in the template polynucleotide labelled as a triangle. The MuA transposase fragments the template double-stranded polynucleotide and inserts the MuA substrates at each side of the point of fragmentation (Step 1). The Uracil-Specific Excision Reagent (USER™) then generates a single nucleotide gap at the location of the single nucleoside that is not present in the template polynucleotide (triangle) which allows the removal of the DNA fragment which contains the single nucleoside that is not present in the template polynucleotide (d) (Step 2). The single stranded DNA gap which is left in the fragmented double-stranded construct is then repaired using a DNA polymerase (e), which fills in the gap with the appropriate complementary nucleotides, and a DNA ligase (f), which ligates the newly synthesised strand to the double stranded construct comprising a single-stranded gap (Step 3).

FIG. 9 shows a method of modifying a template double-stranded polynucleotide (a) by contacting the strand with a MuA transposase (b) and a population of double-stranded hairpin MuA substrates (c). The hairpin MuA substrate contains a single nucleoside that is not present in the template polynucleotide labelled as a triangle. The MuA transposase fragments the template double-stranded polynucleotide and inserts the MuA substrates at each side of the point of fragmentation (Step 1). The Uracil-Specific Excision Reagent (USER™) then generates a single nucleotide gap at the location of the single nucleoside that is not present in the template polynucleotide (triangle) which allows the removal of the DNA fragment which contains the single nucleoside that is not present in the template polynucleotide (d) (Step 2). The single stranded DNA gap which is left in the fragmented double-stranded construct is then repaired using a DNA polymerase (e), which fills in the gap with the appropriate complementary nucleotides, and a DNA ligase (f), which ligates the newly synthesised strand to the double stranded construct comprising a single-stranded gap (Step 3).

FIG. 10 shows a method of modifying a template double-stranded polynucleotide (a) by contacting the strand with a MuA transposase (b) and a population of Y-shaped MuA substrates (c). The Y-shaped MuA substrate contains a single nucleoside that is not present in the template polynucleotide labelled as a triangle. The MuA transposase fragments the template double-stranded polynucleotide and inserts the Y-shaped MuA substrate at each side of the point of fragmentation (Step 1). The Uracil-Specific Excision Reagent (USER™) then generates a single nucleotide gap at the location of the single nucleoside that is not present in the template polynucleotide (triangle) which allows the removal of the DNA fragment which contains the single nucleoside that is not present in the template polynucleotide (d) (Step 2). The single stranded DNA gap which is left in the fragmented double-stranded construct is then repaired using a DNA polymerase (e), which fills in the gap with the appropriate complementary nucleotides, and a DNA ligase (f), which ligates the newly synthesised strand to the double stranded construct comprising a single-stranded gap (Step 3).

FIG. 11 shows a method of modifying a template double-stranded polynucleotide (a) by contacting the strand with a MuA transposase (b) and a population of double-stranded hairpin MuA substrates (d) and Y-shaped MuA substrates (c). Both types of MuA substrate contain a single nucleoside that is not present in the template polynucleotide labelled as a triangle. The MuA transposase fragments the template double-stranded polynucleotide and inserts the MuA substrates at each side of the point of fragmentation (Step 1). The Uracil-Specific Excision Reagent (USER™) then generates a single nucleotide gap at the location of any single nucleosides that are not present in the template polynucleotide (triangle) which allows the removal of the DNA fragments which contain the single nucleoside that is not present in the template polynucleotide (e) (Step 2). The single stranded DNA gap which is left in the fragmented double-stranded construct is then repaired using a DNA polymerase (f), which fills in the gap with the appropriate complementary nucleotides, and a DNA ligase (g), which ligates the newly synthesised strand to the double stranded construct comprising a single-stranded gap (Step 3).

FIG. 12 shows a method of modifying a template double-stranded polynucleotide (a) by contacting the strand with a MuA transposase (b) and a population of double-stranded hairpin MuA substrates (d) and Y-shaped MuA substrates (c). Both types of MuA substrate contain a single nucleoside that is not present in the template polynucleotide labelled as a triangle. The MuA transposase fragments the template double-stranded polynucleotide and inserts the MuA substrates at each side of the point of fragmentation (Step 1). The Uracil-Specific Excision Reagent (USER™) then generates a single nucleotide gap at the location of any single nucleosides that are not present in the template polynucleotide (triangle) which allows the removal of the DNA fragments which contain the single nucleoside that is not present in the template polynucleotide (e) (Step 2). The single stranded DNA gaps which are left in the fragmented double-stranded construct are then repaired using a DNA polymerase (f), which fills in the gaps with the appropriate complementary nucleotides, and a DNA ligase (g), which ligates the newly synthesised strand to the double stranded construct comprising a single-stranded gap (Step 3).

FIG. 13 shows a method of modifying a template double-stranded polynucleotide (a) by contacting the strand with a MuA transposase (b) and a population of double-stranded hairpin MuA substrates (d) and Y-shaped MuA substrates (c). The Y-shaped MuA substrate has an additional polyT leader sequence (show as a dashed wiggly line). Both types of MuA substrate contain a single nucleoside that is not present in the template polynucleotide labelled as a triangle. The MuA transposase fragments the template double-stranded polynucleotide and inserts the MuA substrates at each side of the point of fragmentation (Step 1). The Uracil-Specific Excision Reagent (USER™) then generates a single nucleotide gap at the location of any single nucleosides that are not present in the template polynucleotide (triangle) which allows the removal of the DNA fragments which contain the single nucleoside that is not present in the template polynucleotide (e) (Step 2). The single stranded DNA gaps which are left in the fragmented double-stranded construct are then repaired using a DNA polymerase (f), which fills in the gaps with the appropriate complementary nucleotides, and a DNA ligase (g), which ligates the newly synthesised strand to the double stranded construct comprising a single-stranded gap (Step 3).

FIG. 14 shows a method of modifying a template double-stranded polynucleotide (a) by contacting the strand with a MuA transposase (b) and a population of double-stranded hairpin MuA substrates (c). The hairpin MuA substrate contains a single nucleoside that is not present in the template polynucleotide labelled as a triangle. The MuA transposase fragments the template double-stranded polynucleotide and inserts the MuA substrates at each side of the point of fragmentation (Step 1). The Uracil-Specific Excision Reagent (USER™) then generates a single nucleotide gap at the location of any single nucleosides that are not present in the template polynucleotide (triangle) which allows the removal of the DNA fragments which contain the single nucleoside that is not present in the template polynucleotide (d) (Step 2). The single stranded DNA gaps which are left in the fragmented double-stranded construct are then repaired using a DNA polymerase (f), which fills in the gaps with the appropriate complementary nucleotides, and a DNA ligase (g), which ligates the newly synthesised strand to the double stranded construct comprising a single-stranded gap (Step 3).

FIG. 15 shows the DNA substrate designs for the two MuA substrates used in Example 1 (MuA substrate 1 is labelled A and MuA substrate 2 is labelled B). The dUMP in SEQ ID NO: 26 is highlighted as a star and the C3 spacers are shown as x's.

FIG. 16 shows the lanes in the gel related to the following samples—lane 1=appropriate DNA ladder (Units of the DNA markers is kb), lane 2 corresponded to sample 1 (a control showing the band produced by unmodified SEQ ID NO: 29), lane 3 corresponded to sample 2 (the wild-type MuA substrate (MuA substrate 1, SEQ ID NO: 27 attached to SEQ ID NO: 24 by four C3 spacer units, hybridised to SEQ ID NO: 28) which did not contain at least one nucleoside which was not present in the template polynucleotide (SEQ ID NO: 29)), lane 4 corresponded to sample 3 (the MuA substrate which contained a dUMP (MuA substrate 2, SEQ ID NO: 27 attached to SEQ ID NO: 24 by four C3 spacer units, hybridised to SEQ ID NO: 26) which was not present in the template polynucleotide (SEQ ID NO: 29)), lane 5 corresponded to sample 4 (the broad band on the gel indicated that the MuA transposase was able to insert the wild-type MuA substrate (SEQ ID NO: 27 attached to SEQ ID NO: 24 by four C3 spacer units, hybridised to SEQ ID NO: 28) into the template polynucleotide (SEQ ID NO: 29)), lane 6 corresponded to sample 5 (the broad band on the gel indicated that the MuA transposase was able to insert the MuA substrate which contained a dUMP (SEQ ID NO: 27 attached to SEQ ID NO: 24 by four C3 spacer units, hybridised to SEQ ID NO: 26) into the template polynucleotide (SEQ ID NO: 29)). Lanes 3 and 4 produced a faint band at the bottom of the gel as they were run as additional controls.

FIG. 17 section A) shows the sample preparation process described in Example 2. In step 1 ssDNA fragments were annealed in order to form a model construct (X). Step 2 shows the removal of the dUMP from construct X (labelled as a triangle) using USER™. Finally, in step 3 the gap in construct x was repaired using a DNA polymerase and ligase, to produce a ds-DNA polynucleotide. Section B) shows the control strand, where SEQ ID NO: 30 annealed to SEQ ID NO: 33 its complementary strand.

FIG. 18 shows an Agilent 1000 chip which had the following samples contained in lanes 1-9−lane 1=appropriate DNA ladder, lane 2=control sample a (contained construct Y, no digestion), lane 3=control sample b (contained construct Y after digestion), lane 4=control sample c (contained construct X which had not been exposed to polymerase or ligase, no digestion), lane 5=control sample d (contained construct X which had not been exposed to polymerase or ligase after digestion), lane 6=control sample e (contained construct X which had been exposed to polymerase only, no digestion), lane 7=control sample f (contained construct X which had been exposed to polymerase only, after digestion), lane 8=control sample g (contained construct X which had been exposed to polymerase and ligase, no digestion), lane 9=control sample h (contained construct X which had not been exposed to polymerase and ligase, after digestion). The band marked with a star corresponded to undigested dsDNA constructs and the bands marked with a square corresponded to digested DNA.

FIG. 19 shows a cartoon representation of the sample preparation procedure used in example 3. In step 1 the double-stranded Lambda DNA (SEQ ID NO: 29, labelled a) was contacted with a MuA transposase (b) and a population of double-stranded hairpin MuA substrates (d) and Y-shaped MuA substrates (c). Both types of MuA substrate contained a single dUMP labelled as a triangle. The MuA transposase fragmented the lambda DNA into 5-10 kB fragments and inserted the MuA substrates at each side of the point of fragmentation (Step 1). The Uracil-Specific Excision Reagent (USER™) then generated a single nucleotide gap at the location of any dUMPs (triangle) which allowed the removal of the DNA fragments which contained the dUMP (e) (Step 2). The single stranded DNA gaps which were left in the fragmented double-stranded construct were then repaired using a DNA polymerase (f), which filled in the gaps with the appropriate complementary nucleotides, and a DNA ligase (g), which ligates the newly synthesised strand to the double stranded construct comprising a single-stranded gap (Step 3). In step 4, the resultant Lambda DNA construct (h) then translocated through a nanopore (i) under the control of a helicase enzyme (j).

FIG. 20 (a) shows the hairpin and Y-shaped MuA substrate designs used in Example 3. The dUMP in SEQ ID NO: 36 is highlighted as a triangle and the iSpC3 spacers are shown as x's. FIG. 20 (b) shows the Lambda DNA construct produced during the sample preparation procedure detailed in Example 3. The 5-10 kB fragment of Lambda DNA is labelled X, the fragment of DNA filled in by the polymerase and joined to the rest of the construct by the ligase is labelled y (and is shown as a dotted line) and the iSpC3 spacers are shown as x's. A tether sequence (SEQ ID NO: 39) was hybridised to the DNA construct as shown. Attached at the 3′ end of SEQ ID NO: 39 was six iSp18 spacers attached to two thymine residues and a 3′ cholesterol TEG (shown as a grey circle).

FIG. 21 shows example current traces (y-axis label=Current (pA), x-axis label=Time (s) for both upper and lower traces) of when a helicase (Trwc Cba (SEQ ID NO: 40, labelled A) controlled the translocation of the Lambda DNA construct (shown in FIG. 20 (b) through a nanopore (MS(B1-G75S/G77S/L88N/Q126R)8 MspA (SEQ ID NO: 2 with mutations G75S/G77S/L88N/Q126R)). The upper trace shows helicase controlled DNA movement of the entire lambda DNA construct through the nanopore, the first iSpC3 spacer labelled X1 produced the spike in current labelled 1 and the second iSpC3 spacers labelled X2 produced the spike in current labelled 2. The lower trace shows a zoomed in region of the end of the helicase controlled DNA movement through the nanopore, the third iSpC3 spacer labelled X3 produced the spike in current labelled 3.

FIG. 22 shows a cartoon representation of the sample preparation procedure used in example 3. In step 1 the double-stranded Lambda DNA (SEQ ID NO: 29, labelled a) was contacted with a MuA transposase (b) and a population of double-stranded hairpin 2 MuA substrates (d) and Y-shaped 2 MuA substrates (c). Both types of MuA substrate contained a 5′phosphate (labelled as a circle) and five inosines (labelled as a rectangle). The MuA transposase fragmented the lambda DNA into 5-10 kB fragments and inserted the MuA substrates at each side of the point of fragmentation (Step 1). The nicks which wee left in the fragmented double-stranded construct wee then repaired using a DNA ligase (e), which ligated the strand that contained the inosines to the double stranded Lambda DNA construct (Step 2). In step 3, the resultant Lambda DNA construct (f) then translocated through a nanopore (h) under the control of a helicase enzyme (g).

FIG. 23 (a) shows the hairpin 2 and Y-shaped 2 MuA substrate designs used in Example 4. The 5′ phosphate is labelled as a circle, the inosines in SEQ ID NO: 41 are highlighted as a rectangle and the iSpC3 spacers are shown as x's. FIG. 23 (b) shows the Lambda DNA construct produced during the sample preparation procedure detailed in Example 4. The 5-10 kB fragment of Lambda DNA is labelled X, the inosines which have now been attached to x are labelled as a rectangle and the iSpC3 spacers are shown as x's. A tether sequence (SEQ ID NO: 39) was hybridised to the DNA construct as shown. Attached at the 3′ end of SEQ ID NO: 39 was six iSp18 spacers attached to two thymine residues and a 3′ cholesterol TEG (shown as a grey circle).

FIG. 24 shows example current traces (y-axis label=Current (pA), x-axis label=Time (s) for both upper and lower traces) of when a helicase (Trwc Cba (SEQ ID NO: 40, labelled A) controlled the translocation of the Lambda DNA construct through a nanopore (MS(B1-G75S/G77S/L88N/Q126R)8 MspA (SEQ ID NO: 2 with mutations G75S/G77S/L88N/Q126R)). The upper trace shows helicase controlled DNA movement of the entire lambda DNA construct through the nanopore, the first iSpC3 spacer labelled X1 produced the spike in current labelled 1, the second iSpC3 spacer labelled X2 produced the spike in current labelled 2 and the third iSpC3 spacer labelled X3 produced the spike in current labelled 3. The lower trace shows a zoomed in region of the second half of the helicase controlled DNA movement through the nanopore, the second iSpC3 spacer labelled X2 produced the spike in current labelled 2 and the third iSpC3 spacer labelled X3 produced the spike in current labelled 3.

DESCRIPTION OF THE SEQUENCE LISTING

SEQ ID NO: 1 shows the codon optimised polynucleotide sequence encoding the MS-B1 mutant MspA monomer. This mutant lacks the signal sequence and includes the following mutations: D90N, D91N, D93N, D118R, D134R and E139K.

SEQ ID NO: 2 shows the amino acid sequence of the mature form of the MS-B1 mutant of the MspA monomer. This mutant lacks the signal sequence and includes the following mutations: D90N, D91N, D93N, D118R, D134R and E139K.

SEQ ID NO: 3 shows the polynucleotide sequence encoding one monomer of □-hemolysin-E111N/K147N (α-HL-NN; Stoddart et al., PNAS, 2009; 106(19): 7702-7707).

SEQ ID NO: 4 shows the amino acid sequence of one monomer of α-HL-NN.

SEQ ID NOS: 5 to 7 show the amino acid sequences of MspB, C and D.

SEQ ID NO: 8 shows the polynucleotide sequence encoding the Phi29 DNA polymerase.

SEQ ID NO: 9 shows the amino acid sequence of the Phi29 DNA polymerase.

SEQ ID NO: 10 shows the codon optimised polynucleotide sequence derived from the sbcB gene from E. coli. It encodes the exonuclease I enzyme (EcoExo I) from E. coli.

SEQ ID NO: 11 shows the amino acid sequence of exonuclease I enzyme (EcoExo I) from E. coli.

SEQ ID NO: 12 shows the codon optimised polynucleotide sequence derived from the xthA gene from E. coli. It encodes the exonuclease III enzyme from E. coli.

SEQ ID NO: 13 shows the amino acid sequence of the exonuclease III enzyme from E. coli. This enzyme performs distributive digestion of 5′ monophosphate nucleosides from one strand of double stranded DNA (dsDNA) in a 3′-5′ direction. Enzyme initiation on a strand requires a 5′ overhang of approximately 4 nucleotides.

SEQ ID NO: 14 shows the codon optimised polynucleotide sequence derived from the recJ gene from T. thermophilus. It encodes the RecJ enzyme from T. thermophilus (TthRecJ-cd).

SEQ ID NO: 15 shows the amino acid sequence of the RecJ enzyme from T. thermophilus (TthRecJ-cd). This enzyme performs processive digestion of 5′ monophosphate nucleosides from ssDNA in a 5′-3′ direction. Enzyme initiation on a strand requires at least 4 nucleotides.

SEQ ID NO: 16 shows the codon optimised polynucleotide sequence derived from the bacteriophage lambda exo (redX) gene. It encodes the bacteriophage lambda exonuclease.

SEQ ID NO: 17 shows the amino acid sequence of the bacteriophage lambda exonuclease. The sequence is one of three identical subunits that assemble into a trimer. The enzyme performs highly processive digestion of nucleotides from one strand of dsDNA, in a 5′-3′ direction (neb.com/nebecomm/products/productM0262.asp). Enzyme initiation on a strand preferentially requires a 5′ overhang of approximately 4 nucleotides with a 5′ phosphate.

SEQ ID NO: 18 shows the amino acid sequence of Hel308 Mbu.

SEQ ID NO: 19 shows the Hel308 motif of Hel308 Csy.

SEQ ID NO: 20 shows the amino acid sequence of Hel308 Tga.

SEQ ID NO: 21 shows the amino acid sequence of Hel308 Mhu.

SEQ ID NO: 22 shows the amino acid sequence of TraI Eco.

SEQ ID NO: 23 shows the amino acid sequence of XPD Mbu.

SEQ ID NO: 24 shows the polynucleotide sequence of the double stranded portion of a MuA substrate of the invention. In Example 1 this sequence is attached to one end of a chain of four C3 spacer units by its 5′ end. The opposite end of the chain of C3 spacer units is attached to the 3′ end of SEQ ID NO: 27.

SEQ ID NO: 25 shows the polynucleotide sequence of the double stranded portion of a MuA substrate of the invention. This sequence is complementary to SEQ ID NO: 24 except that it contains a U at the 3′ end.

SEQ ID NO: 26 shows polynucleotide sequence of the overhang strand of the double stranded MuA substrate of the invention. In Example 1 this sequence has a phosphate group attached to the 5′ end of the sequence.

SEQ ID NO: 27 shows one of the polynucleotide sequences used in Example 1. It is attached to one end of a chain of four C3 spacer units by its 3′ end. The opposite end of the chain of C3 spacer units is attached to the 5′ end of SEQ ID NO: 24.

SEQ ID NO: 28 shows one of the polynucleotide sequences used in Example 1.

SEQ ID NO: 29 shows the polynucleotide sequence of the Enterobacteria phage λ. The sequence contains an additional 12 base overhang attached at the 5′ end of the template strand. The sequence shown here is that of the template strand only.

SEQ ID NO: 30 shows one of the polynucleotide sequences used in Example 2.

SEQ ID NO: 31 shows one of the polynucleotide sequences used in Example 2.

SEQ ID NO: 32 shows one of the polynucleotide sequences used in Example 2. It has a phosphate group attached to the 5′ end of the sequence.

SEQ ID NO: 33 shows the complementary polynucleotide sequence to SEQ ID NO: 30 used in Example 2.

SEQ ID NO: 34 shows part of a polynucleotide sequence used in Example 3 and 4. SEQ ID NO: 34 is attached at its 3′ end to the 5′ end of SEQ ID NO: 35 by four iSpC3 spacer units.

SEQ ID NO: 35 shows part of a polynucleotide sequence used in Example 3 and 4. In the Y-shaped MuA substrate of Examples 3 and 4 SEQ ID NO: 35 is attached at its 5′ end to the 3′ end of SEQ ID NO: 34 by four iSpC3 spacer units. In the hairpin loop of Example 3 SEQ ID NO: 35 is attached at its 5′ end to the 3′ end of SEQ ID NO: 36 by four iSpC3 spacers. In the hairpin MuA substrate of Example 4 SEQ ID NO: 35 is attached at its 5′ end to the 3′ end of SEQ ID NO:

41 by four iSpC3 spacers.

SEQ ID NO: 36 shows part of a polynucleotide sequence used in Example 3. In the Y-shaped MuA substrate SEQ ID NO: 36 is attached at its 3′ end to the 5′ end of SEQ ID NO: 37 by four iSpC3 spacer units. In the hairpin MuA substrate SEQ ID NO: 36 is attached at the 3′ end to the 5′ end of SEQ ID NO: 35 by four iSpC3 spacers.

SEQ ID NO: 37 shows part of a polynucleotide sequence used in Example 3 and 4. In Example 3 SEQ ID NO: 37 is attached at its 5′ end to the 3′ end of SEQ ID NO: 36 by four iSpC3 spacer units. In Example 4 SEQ ID NO: 37 is attached at its 5′ end to the 3′ end of SEQ ID NO: 41 by four iSpC3 spacer units.

SEQ ID NO: 38 shows part of a polynucleotide sequence used in Example 3.

SEQ ID NO: 39 shows a polynucleotide sequence used in Example 3 and 4 which at the 3′ end of the sequence has six iSp18 spacers attached to two thymine residues and a 3′ cholesterol TEG.

SEQ ID NO: 40 shows the amino acid sequence of the Trwc Cba helicase.

SEQ ID NO: 41 shows part of a polynucleotide sequence used in Example 4. In the Y-shaped MuA substrate SEQ ID NO: 41 is attached at its 3′ end to the 5′ end of SEQ ID NO: 37 by four iSpC3 spacer units. In the hairpin MuA substrate SEQ ID NO: 41 is attached at its 3′ end to the 5′ end of SEQ ID NO: 35 by four iSpC3 spacer units. This sequence has a phosphate attached to its 5′ end and 5 deoxyinosines at positions 1 to 5 indicated in the sequence by an I.

DETAILED DESCRIPTION OF THE INVENTION

It is to be understood that different applications of the disclosed products and methods may be tailored to the specific needs in the art. It is also to be understood that the terminology used herein is for the purpose of describing particular embodiments of the invention only, and is not intended to be limiting.

In addition as used in this specification and the appended claims, the singular forms “a”, “an”, and “the” include plural referents unless the content clearly dictates otherwise. Thus, for example, reference to “a polynucleotide” includes “polynucleotides”, reference to “a substrate” includes two or more such substrates, reference to “a transmembrane protein pore” includes two or more such pores, and the like.

All publications, patents and patent applications cited herein, whether supra or infra, are hereby incorporated by reference in their entirety.

Modification Method of the Invention

The present invention provides a method of modifying a template polynucleotide. The template may be modified for any purpose. The method is preferably for modifying a template polynucleotide for characterisation, such as for strand sequencing. The template polynucleotide is typically the polynucleotide that will ultimately be characterised, or sequenced, in accordance with the invention. This is discussed in more detail below.

The method involves the formation of a plurality of modified double stranded polynucleotides. These modified double stranded polynucleotides are typically easier to characterise than the template polynucleotide, especially using strand sequencing. The plurality of modified double stranded polynucleotides may themselves be characterised in order to facilitate the characterisation of the template polynucleotide. For instance, the sequence of the template polynucleotide can be determined by sequencing each of the modified double stranded polynucleotides.

The modified double stranded polynucleotides are shorter than the template polynucleotide and so it is more straightforward to characterise them using strand sequencing.

The modified double strand polynucleotides can be selectively labelled by including the labels in the MuA substrates. Suitable labels include, but are not limited to, calibration sequences, coupling moieties and adaptor bound enzymes.

In some embodiments, the method introduces into the double stranded polynucleotides modifications which facilitate their characterisation using strand sequencing. It is well-established that coupling a polynucleotide to the membrane containing the nanopore lowers by several orders of magnitude the amount of polynucleotide required to allow its characterisation or sequencing. This is discussed in International Application No. PCT/GB2012/051191 (published as WO 2012/164270). The method of the invention allows the production of a plurality of double stranded polynucleotides each of which include a means for coupling the polynucleotides to a membrane. This is discussed in more detail below.

The characterisation of double stranded polynucleotides using a nanopore typically requires the presence of a leader sequence designed to preferentially thread into the nanopore. The method of the invention allows the production of a plurality of double stranded polynucleotides each of which include a single stranded leader sequence. This is discussed in more detail below.

It is also well established that linking the two strands of a double stranded polynucleotide by a bridging moiety, such as hairpin loop, allows both strands of the polynucleotide to be characterised or sequenced by a nanopore. This is advantageous because it doubles the amount of information obtained from a single double stranded polynucleotide. Moreover, because the sequence in the template complement strand is necessarily orthogonal to the sequence of the template strand, the information from the two strands can be combined informatically. Thus, this mechanism provides an orthogonal proof-reading capability that provides higher confidence observations. This is discussed in International Application No. PCT/GB2012/051786 (published as WO 2013/014451). The method of the invention allows the production of a plurality of modified double stranded polynucleotides in which the two strands of each polynucleotide are linked using a hairpin loop.

Template Polynucleotide

The method of the invention modifies a template double stranded polynucleotide, preferably for characterisation. The template polynucleotide is typically the polynucleotide that will ultimately be characterised, or sequenced, in accordance with the invention. It may also be called the target double stranded polynucleotide or the double stranded polynucleotide of interest.

A polynucleotide, such as a nucleic acid, is a macromolecule comprising two or more nucleotides. The polynucleotide or nucleic acid may comprise any combination of any nucleotides. The nucleotides can be naturally occurring or artificial. One or more nucleotides in the template polynucleotide can be oxidized or methylated. One or more nucleotides in the template polynucleotide may be damaged. For instance, the polynucleotide may comprise a pyrimidine dimer. Such dimers are typically associated with damage by ultraviolet light and are the primary cause of skin melanomas. One or more nucleotides in the template polynucleotide may be modified, for instance with a label or a tag. Suitable labels are described below. The template polynucleotide may comprise one or more spacers.

A nucleotide typically contains a nucleobase, a sugar and at least one phosphate group. The nucleobase and sugar form a nucleoside.

The nucleobase is typically heterocyclic. Nucleobases include, but are not limited to, purines and pyrimidines and more specifically adenine (A), guanine (G), thymine (T), uracil (U) and cytosine (C).

The sugar is typically a pentose sugar. Nucleotide sugars include, but are not limited to, ribose and deoxyribose. The sugar is preferably a deoxyribose.

The template double stranded polynucleotide preferably comprises the following nucleosides: deoxyadenosine (dA), deoxyuridine (dU) and/or thymidine (dT), deoxyguanosine (dG) and deoxycytidine (dC).

The nucleotide is typically a ribonucleotide or deoxyribonucleotide. The nucleotide is preferably a deoxyribonucleotide. The nucleotide typically contains a monophosphate, diphosphate or triphosphate. Phosphates may be attached on the 5′ or 3′ side of a nucleotide.

Nucleotides include, but are not limited to, adenosine monophosphate (AMP), guanosine monophosphate (GMP), thymidine monophosphate (TMP), uridine monophosphate (UMP), 5-methylcytidine monophosphate, 5-hydroxymethylcytidine monophosphate, cytidine monophosphate (CMP), cyclic adenosine monophosphate (cAMP), cyclic guanosine monophosphate (cGMP), deoxyadenosine monophosphate (dAMP), deoxyguanosine monophosphate (dGMP), deoxythymidine monophosphate (dTMP), deoxyuridine monophosphate (dUMP) and deoxycytidine monophosphate (dCMP). The nucleotides are preferably selected from AMP, TMP, GMP, CMP, UMP, dAMP, dTMP, dGMP, dCMP and dUMP. The nucleotides are most preferably selected from dAMP, dTMP, dGMP, dCMP and dUMP.

The template double stranded polynucleotide preferably comprises the following nucleotides: dAMP, dUMP and/or dTMP, dGMP and dCMP.

A nucleotide may be abasic (i.e. lack a nucleobase). A nucleotide may also lack a nucleobase and a sugar (i.e. is a C3 spacer).

The nucleotides in the template polynucleotide may be attached to each other in any manner. The nucleotides are typically attached by their sugar and phosphate groups as in nucleic acids. The nucleotides may be connected via their nucleobases as in pyrimidine dimers.

The template polynucleotide is double stranded. The template polynucleotide may contain some single stranded regions, but at least a portion of the template polynucleotide is double stranded.

The template polynucleotide can be a nucleic acid, such as deoxyribonucleic acid (DNA) or ribonucleic acid (RNA). The template polynucleotide can comprise one strand of RNA hybridised to one strand of DNA. The polynucleotide may be any synthetic nucleic acid known in the art, such as peptide nucleic acid (PNA), glycerol nucleic acid (GNA), threose nucleic acid (TNA), locked nucleic acid (LNA) or other synthetic polymers with nucleotide side chains.

The template polynucleotide can be any length. For example, the polynucleotide can be at least 10, at least 50, at least 100, at least 150, at least 200, at least 250, at least 300, at least 400 or at least 500 nucleotide pairs in length. The polynucleotide can be 1000 or more nucleotide pairs, 5000 or more nucleotide pairs in length or 100000 or more nucleotide pairs in length.

The template polynucleotide is typically present in any suitable sample. The invention is typically carried out on a sample that is known to contain or suspected to contain the template polynucleotide. Alternatively, the invention may be carried out on a sample to confirm the identity of one or more template polynucleotides whose presence in the sample is known or expected.

The sample may be a biological sample. The invention may be carried out in vitro on a sample obtained from or extracted from any organism or microorganism. The organism or microorganism is typically archaeal, prokaryotic or eukaryotic and typically belongs to one of the five kingdoms: plantae, animalia, fungi, monera and protista. The invention may be carried out in vitro on a sample obtained from or extracted from any virus. The sample is preferably a fluid sample. The sample typically comprises a body fluid of the patient. The sample may be urine, lymph, saliva, mucus or amniotic fluid but is preferably blood, plasma or serum. Typically, the sample is human in origin, but alternatively it may be from another mammal animal such as from commercially farmed animals such as horses, cattle, sheep or pigs or may alternatively be pets such as cats or dogs. Alternatively a sample of plant origin is typically obtained from a commercial crop, such as a cereal, legume, fruit or vegetable, for example wheat, barley, oats, canola, maize, soya, rice, bananas, apples, tomatoes, potatoes, grapes, tobacco, beans, lentils, sugar cane, cocoa, cotton.

The sample may be a non-biological sample. The non-biological sample is preferably a fluid sample. Examples of a non-biological sample include surgical fluids, water such as drinking water, sea water or river water, and reagents for laboratory tests.

The sample is typically processed prior to being used in the invention, for example by centrifugation or by passage through a membrane that filters out unwanted molecules or cells, such as red blood cells. The sample may be measured immediately upon being taken. The sample may also be typically stored prior to assay, preferably below −70° C.

MuA and Conditions

The template polynucleotide is contacted with a MuA transposase. This contacting occurs under conditions which allow the transposase to function, i.e. to fragment the template polynucleotide and to ligate MuA substrates to the one or both ends of the fragments. MuA transposase is commercially available, for instance from Thermo Scientific (Catalogue Number F-750 C, 20 μL (1.1 μg/μL)). Conditions under which MuA transposase will function are known in the art. Suitable conditions are described in the Examples.

Population of Substrates

The template polynucleotide is contacted with a population of double stranded MuA substrates. The double stranded substrates are polynucleotide substrates and may be formed from any of the nucleotides or nucleic acids discussed above. The substrates are typically formed from the same nucleotides as the template polynucleotide, except for the universal nucleotides or at least one nucleotide which comprises a nucleoside that is not present in the template polynucleotide.

The population of substrates is typically homogenous (i.e. typically contains a plurality of identical substrates). The population of substrates may be heterogeneous (i.e. may contain a plurality of different substrates).

Suitable substrates for a MuA transposase are known in the art (Saariaho and Savilahti, Nucleic Acids Research, 2006; 34(10): 3139-3149 and Lee and Harshey, J. Mol. Biol., 2001; 314: 433-444).

Each substrate typically comprises a double stranded portion which provides its activity as a substrate for MuA transposase. The double stranded portion is typically the same in each substrate. The population of substrates may comprise different double stranded portions.

The double stranded portion in each substrate is typically at least 50 nucleotide pairs in length, such as at least 55, at least 60 or at least 65 nucleotide pairs in length. The double stranded portion in each substrate preferably comprises a dinucleotide comprising deoxycytidine (dC) and deoxyadenosine (dA) at the 3′ end of each strand. The dC and dA are typically in different orientations in the two strands of the double stranded portion, i.e. one strand has dC/dA and the other strand has dA/dC at the 3′ end when reading from 5′ to 3′.

One strand of the double stranded portion preferably comprises the sequence shown in SEQ ID NO: 24 and the other strand of the double stranded portion preferably comprises a sequence which is complementary to the sequence shown in SEQ ID NO: 24.

Universal Nucleotides

In one embodiment, the method comprises contacting the template polynucleotide with a population of double stranded MuA substrates each comprising at least one overhang of universal nucleotides. Each substrate preferably comprises a double stranded portion which comprises the sequence shown in SEQ ID NO: 24 hybridised to a sequence which is complementary to the sequence shown in SEQ ID NO: 24. The at least one overhang is preferably at the 5′ end of the sequence which is complementary to the sequence shown in SEQ ID NO: 24.

Each substrate comprises at least one overhang of universal nucleotides. The overhang consists of universal nucleotides. There may be an overhang of universal nucleotides at one or both ends of each substrate. If there is an overhang at both ends of a substrate, each overhang is typically on different strands of the double stranded polynucleotide portion. Overhangs are preferably located at the 5′ end of a strand of the double stranded portion.

Each substrate preferably comprises only one overhang. The only one overhang is preferably at the 5′ end of one strand of the double stranded portion.

The overhang may be at least 3, at least 4, at least 5, at least 6 or at least 7 nucleotides in length. The overhang is preferably 5 nucleotides in length.

A universal nucleotide is one which will hybridise to some degree to all of the nucleotides in the template polynucleotide. A universal nucleotide is preferably one which will hybridise to some degree to nucleotides comprising the nucleosides adenosine (A), thymine (T), uracil (U), guanine (G) and cytosine (C). The universal nucleotide may hybridise more strongly to some nucleotides than to others. For instance, a universal nucleotide (I) comprising the nucleoside, 2′-deoxyinosine, will show a preferential order of pairing of I-C>I-A>I-G approximately =I-T. For the purposes of the invention, it is only necessary that the universal nucleotide used in the oligomers hybridises to all of the nucleotides in the template polynucleotide.

The universal nucleotide preferably comprises one of the following nucleobases: hypoxanthine, 4-nitroindole, 5-nitroindole, 6-nitroindole, 3-nitropyrrole, nitroimidazole, 4-nitropyrazole, 4-nitrobenzimidazole, 5-nitroindazole, 4-aminobenzimidazole or phenyl (C6-aromatic ring. The universal nucleotide more preferably comprises one of the following nucleosides: 2′-deoxyino sine, inosine, 7-deaza-2′-deoxyinosine, 7-deaza-inosine, 2-aza-deoxyinosine, 2-aza-inosine, 4-nitroindole 2′-deoxyribonucleoside, 4-nitroindole ribonucleoside, 5-nitroindole 2′-deoxyribonucleoside, 5-nitroindole ribonucleoside, 6-nitroindole 2′-deoxyribonucleoside, 6-nitroindole ribonucleoside, 3-nitropyrrole 2′-deoxyribonucleoside, 3-nitropyrrole ribonucleoside, an acyclic sugar analogue of hypoxanthine, nitroimidazole 2′-deoxyribonucleoside, nitroimidazole ribonucleoside, 4-nitropyrazole 2′-deoxyribonucleoside, 4-nitropyrazole ribonucleoside, 4-nitrobenzimidazole 2′-deoxyribonucleoside, 4-nitrobenzimidazole ribonucleoside, 5-nitroindazole 2′-deoxyribonucleoside, 5-nitroindazole ribonucleoside, 4-aminobenzimidazole 2′-deoxyribonucleoside, 4-aminobenzimidazole ribonucleoside, phenyl C-ribonucleoside or phenyl C-2′-deoxyribosyl nucleoside. The universal nucleotide is most preferably comprises 2′-deoxyinosine.

The universal nucleotides in each overhang may be different from one another. The universal nucleotides in each overhang are preferably the same. All of the universal nucleotides in the population of substrates are preferably the same universal nucleotide.

The method of the invention preferably comprises

(a) contacting the template polynucleotide with a MuA transposase and a population of double stranded MuA substrates each comprising at least one overhang of universal nucleotides such that the transposase fragments the template polynucleotide into fragments having an overhang at one or both ends and ligates a substrate strand without an overhang to an overhang at one or both ends of the double stranded fragments and thereby producing a plurality of fragment/substrate constructs;

(b) allowing the substrate overhangs to hybridise to the fragments overhangs in the constructs; and

(b) ligating the substrate overhangs to the adjacent fragment strands in the constructs and thereby producing a plurality of modified double stranded polynucleotides.

The overhang(s) of universal nucleotides may further comprise a reactive group, preferably at the 5′ end. The reactive group may be used to ligate the overhangs to the fragments in the constructs as discussed below. The reactive group may be used to ligate the fragments to the overhangs using click chemistry. Click chemistry is a term first introduced by Kolb et al. in 2001 to describe an expanding set of powerful, selective, and modular building blocks that work reliably in both small- and large-scale applications (Kolb H C, Finn, M G, Sharpless K B, Click chemistry: diverse chemical function from a few good reactions, Angew. Chem. Int. Ed. 40 (2001) 2004-2021). They have defined the set of stringent criteria for click chemistry as follows: “The reaction must be modular, wide in scope, give very high yields, generate only inoffensive by-products that can be removed by nonchromatographic methods, and be stereospecific (but not necessarily enantioselective). The required process characteristics include simple reaction conditions (ideally, the process should be insensitive to oxygen and water), readily available starting materials and reagents, the use of no solvent or a solvent that is benign (such as water) or easily removed, and simple product isolation. Purification if required must be by nonchromatographic methods, such as crystallization or distillation, and the product must be stable under physiological conditions”.

Suitable examples of click chemistry include, but are not limited to, the following:

-   -   (a) copper-free variant of the 1,3 dipolar cycloaddition         reaction, where an azide reacts with an alkyne under strain, for         example in a cyclooctane ring;     -   (b) the reaction of an oxygen nucleophile on one linker with an         epoxide or aziridine reactive moiety on the other; and     -   (c) the Staudinger ligation, where the alkyne moiety can be         replaced by an aryl phosphine, resulting in a specific reaction         with the azide to give an amide bond.

Any reactive group may be used in the invention. The reactive group may be one that is suitable for click chemistry. The reactive group may be any of those disclosed in International Application No. PCT/GB10/000132 (published as WO 2010/086602), particularly in Table 3 of that application.

In a further embodiment, the modification method uses a MuA transposase and a population of MuA substrates each comprising at least one overhang comprising a reactive group. The overhang(s) may be any length and may comprise any combination of any nucleotide(s). Suitable lengths and nucleotides are disclosed above. Suitable reactive groups are discussed above. Accordingly, the invention provides a method for modifying a template double stranded polynucleotide, comprising:

(a) contacting the template polynucleotide with a MuA transposase and a population of double stranded MuA substrates each comprising at least one overhang comprising a reactive group such that the transposase fragments the template polynucleotide and ligates a substrate to one or both ends of the double stranded fragments and thereby producing a plurality of fragment/substrate constructs; and

(b) ligating the overhangs to the fragments in the constructs using the reactive group and thereby producing a plurality of modified double stranded polynucleotides.

Nucleosides that are not Present in the Template Polynucleotide

In one embodiment, the method comprises contacting the template polynucleotide with a population of double stranded MuA substrates each comprising (i) at least one overhang and (ii) at least one nucleotide in the same strand as the at least one overhang which comprises a nucleoside that is not present in the template polynucleotide.

As discussed above, the double stranded portion in each substrate preferably comprises a dinucleotide comprising deoxycytidine (dC) and deoxyadenosine (dA) at the 3′ end of each strand. In some embodiments, one or both of the nucleotides in the dC and dA dinucleotide of one strand may be replaced with a nucleotide comprising a nucleoside that is not present in the template polynucleotide as discussed below. In a preferred embodiment, the template polynucleotide comprises deoxyadenosine (dA), thymidine (dT), deoxyguanosine (dG) and deoxycytidine (dC), but not deoxyuridine (dU) and the dA in the dC and dA dinucleotide of one strand is replaced with a nucleotide comprising deoxyuridine (dU). This is exemplified below.

One strand of the double stranded portion preferably comprises the sequence shown in SEQ ID NO: 24 and the other strand of the double stranded portion preferably comprises a sequence which is complementary to the sequence shown in SEQ ID NO: 24 and which is modified to include at least one nucleotide that is not present in the template polynucleotide. This “other strand” further comprises the overhang. In a more preferred embodiment, one strand of the double stranded portion comprises the sequence shown in SEQ ID NO: 24 and the other strand of the double stranded portion comprises the sequence shown in SEQ ID NO: 25 (see below). In SEQ ID NO: 25, the dA in the dC and dA dinucleotide at the 3′ end had been replaced with dU. This double stranded portion (shown below) may be used when the template polynucleotide comprises deoxyadenosine (dA), thymidine (dT), deoxyguanosine (dG) and deoxycytidine (dC), but not deoxyuridine (dU).

(SEQ ID NO: 24) 5′-GTTTTCGCATTTATCGTGAAACGCTTTCGCGTTTTTCGTGCGCCGCT TCA-3′ (SEQ ID NO: 25) 3′-CAAAAGCGTAAATAGCACTTTGCGAAAGCGCAAAAAGCACGCGGCGA AG U -5′

Each substrate comprises at least one overhang. The overhang is typically a nucleotide overhang. There may be an overhang at one or both ends of each substrate. If there is an overhang at both ends of a substrate, each overhang is typically on different strand of the double stranded polynucleotide portion and each strand typically comprises a nucleoside that is not present in the template polynucleotide. Overhangs are preferably located at the 5′ end of a strand of the double stranded portion.

Each substrate preferably comprises only one overhang. The only one overhang is preferably at the 5′ end of one strand of the double stranded portion. This is exemplified below.

The overhang may be at least 3, at least 4, at least 5, at least 6 or at least 7 nucleotides in length. The overhang is preferably 4 nucleotides in length. The overhang may comprise any of the nucleotides discussed above.

Each substrate comprises at least one nucleotide in the same strand as the overhang which comprises a nucleoside that is not present in the template polynucleotide. Such nucleotide(s) may be present in one or both strands of the substrates as discussed above. Each substrate may comprise any number of nucleotides which comprise a nucleoside that is not present in the template polynucleotide, such as 2, 3, 4, 5, 6, 7, 8, 9, 10 or more. If a substrate comprises more than one nucleotide that is not present in the template polynucleotide, those nucleotides are typically the same, but may be different.

If the template polynucleotide comprises deoxyadenosine (dA), thymidine (dT), deoxyguanosine (dG) and deoxycytidine (dC) but not deoxyuridine (dU), the nucleoside that is not present in the template polynucleotide is preferably deoxyuridine (dU).

In a preferred embodiment, one strand of the double stranded portion comprises the sequence shown in SEQ ID NO: 24 and the other strand of the double stranded portion comprises the sequence shown in SEQ ID NO: 25 (see above). In SEQ ID NO: 25, the dA in the dC and dA dinucleotide at the 3′ end had been replaced with dU. The overhang is at the 5′ end of SEQ ID NO: 25, i.e. attached to the U.

In a most preferred embodiment, one strand of the substrate comprises the sequence shown in SEQ ID NO: 24 and the other strand of the substrate comprises the sequence shown in SEQ ID NO: 26 (see below). This substrate (shown below) may be used when the template polynucleotide comprises deoxyadenosine (dA), thymidine (dT), deoxyguanosine (dG) and deoxycytidine (dC), but not deoxyuridine (dU).

(SEQ ID NO: 24) 5′-GTTTTCGCATTTATCGTGAAACGCTTTCGCGTTTTTCGTGCGCCGCT TCA-3′ (SEQ ID NO: 26) 3′-CAAAAGCGTAAATAGCACTTTGCGAAAGCGCAAAAAGCACGCGGCGA AG U CTAG-5′ 

If the template polynucleotide comprises deoxyadenosine (dA), deoxyuridine (dU), deoxyguanosine (dG) and deoxycytidine (dC) but not thymidine (dT), the nucleoside that is not present in the template polynucleotide is preferably thymidine (dT).

The nucleoside that is not present in the template polynucleotide is preferably abasic, adenosine (A), uridine (U), 5-methyluridine (m⁵U), cytidine (C) or guanosine (G) or preferably comprises urea, 5, 6 dihydroxythymine, thymine glycol, 5-hydroxy-5 methylhydanton, uracil glycol, 6-hydroxy-5, 6-dihdrothimine, methyltartronylurea, 7, 8-dihydro-8-oxoguanine (8-oxoguanine), 8-oxoadenine, fapy-guanine, methy-fapy-guanine, fapy-adenine, aflatoxin B1-fapy-guanine, 5-hydroxy-cytosine, 5-hydroxy-uracil, 3-methyladenine, 7-methylguanine, 1,N6-ethenoadenine, hypoxanthine, 5-hydroxyuracil, 5-hydroxymethyluracil, 5-formyluracil or a cis-syn-cyclobutane pyrimidine dimer.

The at least one nucleotide is preferably 10 nucleotides or fewer from the overhang, such as 9, 8, 7, 6, 5, 4, 3, 2, 1 or 0 nucleotides from the overhang. In other words, the at least one nucleotide is preferably at any of positions A to K in the Example below. The at least one nucleotide is preferably 0 nucleotides from the overhang (i.e. is adjacent to the overhang). In other words, the at least one nucleotide is preferably at position K in the Example below.

ABCDEFGHIJKXXXX XXXXXXXXXXX

The at least one nucleotide may be the first nucleotide in the overhang. In other words, the at least one nucleotide may be at position A in the Example below.

XXXXXXXXXXAXXX XXXXXXXXXX

All of the nucleotides in the overhang may comprise a nucleoside that is not present in the template polynucleotide. A person skilled in the art is capable of designing suitable substrates.

The method of the invention preferably comprises

(a) contacting the template polynucleotide with a MuA transposase and a population of double stranded MuA substrates each comprising (i) at least one overhang and (ii) at least one nucleotide in the same strand as the at least one overhang which comprises a nucleoside that is not present in the template polynucleotide such that the transposase fragments the template polynucleotide into fragments having an overhang at one or both ends and ligates a substrate strand without an overhang to an overhang at one or both ends of the double stranded fragments and thereby producing a plurality of fragment/substrate constructs;

(b) removing the overhangs from the constructs by selectively removing the at least one nucleotide and thereby producing a plurality of double stranded constructs comprising single stranded gaps; and

(c) repairing the single stranded gaps in the constructs and thereby producing a plurality of modified double stranded polynucleotides.

Preferred Substrates

In a preferred embodiment, each substrate comprises an overhang at one end and a leader sequence and/or a means for coupling to a membrane at the other. If a substrate comprises a leader sequence and a coupling means, each is typically located on a different strand of the double stranded portion. This results in the production of a plurality of modified double stranded polynucleotides which comprise the leader sequence and/or the means for coupling. Such polynucleotides have the advantages discussed above.

The leader sequence is typically a polynucleotide such as DNA or RNA, a modified polynucleotide (such as abasic DNA), PNA, LNA, PEG or a polypeptide. The leader is preferably a polynucleotide and is more preferably a single stranded polynucleotide. The leader sequence can be any of the polynucleotides discussed above. The single stranded leader sequence is most preferably a single strand of DNA. The leader sequence typically comprises nucleotides that are present in the template polynucleotide. The leader sequence typically does not comprise the nucleoside whose selective removal is used to remove the overhangs in accordance with the invention. The leader sequence can be any length, but is typically 27 to 150 nucleotides in length, such as from 50 to 150 nucleotides in length.

Means for coupling the modified double stranded polynucleotides to a membrane are discussed in more detail below.

Each substrate preferably comprises an overhang at one end and a hairpin loop at the other end. This results in the production of modified double stranded polynucleotides in step (c) which are linked by the hairpin loop at one or both ends. An example of this is shown in FIGS. 2, 7, 9 and 14. A person skilled in the art is capable of preparing hairpin loops using any of the nucleotides discussed above. The hairpin loop typically comprises the same nucleotides as in the template polynucleotide. The hairpin loop typically does not comprise the nucleoside whose selective removal is used to remove the overhangs in accordance with the invention.

Alternatively, each substrate is preferably a Y substrate with an overhang at one end and a region that is not complementary at the other end. The non-complementary region gives the substrate its Y shape since the two strands typically do not hybridise to each other unlike the double stranded portion. The use of Y substrates results in the production of modified double stranded polynucleotides in step (c) which have the non-complementary region at one or both ends. An example of this is shown in FIGS. 3 and 10. The non-complementary region may comprise a leader sequence in one strand and a coupling means in other strand as discussed above.

Alternatively, in a preferred embodiment, a proportion of the substrates in the population comprise an overhang at one end and a hairpin loop at the other end and a proportion of the substrates in the population are Y substrates with an overhang at one end and a region that is not complementary at the other end. This results in at least some of the modified double stranded polynucleotides produced in step (c) having the hairpin loop at one end and the non-complementary region at the other end. An example of this is shown in FIGS. 5 and 12.

Alternatively, in a preferred embodiment, a proportion of the substrates in the population comprise an overhang at one end and a hairpin loop at the other end and a proportion of the substrates in the population comprise an overhang at one end and a leader sequence or coupling means at the other end. This results in at least some of the modified double stranded polynucleotides produced in step (c) having the hairpin loop at one end and the leader sequence or coupling means at the other end. An example of this is shown in FIGS. 6 and 13 (where the leader sequence is shown as a dashed wiggly line).

In the most preferred embodiment, a proportion of the substrates in the population comprise an overhang at one end and a hairpin loop at the other end and a proportion of the substrates in the population comprise an overhang at one end and a leader sequence and a coupling means at the other end. This results in at least some of the modified double stranded polynucleotides produced in step (c) having the hairpin loop at one end and the leader sequence and coupling means at the other end. These modified double stranded polynucleotides are particularly useful for characterisation because they can be coupled to the membrane in which the nanopore is located, have a leader sequence which facilitates the threading of the polynucleotides into the nanopore and both strands can be characterised around the hairpin loop.

In all of the embodiments above, the proportion of one type of substrate may be any proportion, such as at least about 5%, at least about 10%, at least about 20%, at least about 30%, at least about 40%, at least about 50%, at least about 60%, at least about 70%, at least about 80%, at least about 90% or at least about 95%. The remaining proportion of substrates in the population is typically the other type of substrate. For instance, the population may comprise about 40% of the substrates comprising a hairpin loop and about 60% of the Y substrates. The proportion of both types of substrate is preferably about 50%.

Each substrate may comprise a selectable binding moiety. A selectable binding moiety is a moiety that can be selected on the basis of its binding properties. Hence, a selectable binding moiety is preferably a moiety that specifically binds to a surface. A selectable binding moiety specifically binds to a surface if it binds to the surface to a much greater degree than any other moiety used in the invention. In preferred embodiments, the moiety binds to a surface to which no other moiety used in the invention binds.

Suitable selective binding moieties are known in the art. Preferred selective binding moieties include, but are not limited to, biotin, a nucleic acid sequence, antibodies, antibody fragments, such as Fab and ScSv, antigens, nucleic acid binding proteins, poly histidine tails and GST tags. The most preferred selective binding moieties are biotin and a selectable nucleic acid sequence. Biotin specifically binds to a surface coated with avidins. Selectable nucleic acid sequences specifically bind (i.e. hybridize) to a surface coated with homologous sequences. Alternatively, selectable nucleic acid sequences specifically bind to a surface coated with nucleic acid binding proteins.

Ligating the Overhangs

In those embodiments in which the MuA substrates comprise overhangs of universal nucleotides, the method comprises ligating the overhangs to the fragments in the constructs. This may be done using any method of ligating nucleotides known in the art. For instance, it may be done using a ligase, such as a DNA ligase. Alternatively, if the overhangs comprise a reactive group, the reactive group may be used to ligate the overhangs to the fragments in the constructs. For instance, a nucleotide comprising a complementary reactive group may be attached to the fragments and the two reactive groups may be reacted together to ligate the overhangs to the fragments. Click chemistry may be used as discussed above.

Selective Removal

Methods are known in the art for selectively removing the nucleotide(s) which comprise(s) a nucleoside that is not present in the template polynucleotide from the ligated constructs. Nucleotides are selectively removed if they are removed (or excised) from the ligated constructs, but the other nucleotides in the ligated constructs (i.e. those comprising different nucleosides) are not removed (or excised).

Nucleotides comprising deoxyuridine (dU) may be selectively removed using Uracil-Specific Excision Reagent (USER®), which is a mixture of Uracil DNA glycosylase (UDG) and the DNA glycosylase-lyase Endonuclease VIII.

Selective removal of the nucleotide(s) which comprise(s) a nucleoside that is not present in the template polynucleotide from the ligated constructs removes the overhang(s) from the constructs as shown in the Figures. Removal of the overhangs results in a plurality of double stranded constructs comprising single stranded gaps as shown in the FIGS. 8 to 14.

Repairing the Gaps

Methods are known in the art for repairing the single stranded gaps in the double stranded constructs. For instance, the gaps can be repaired using a polymerase and a ligase, such as DNA polymerase and a DNA ligase. Alternatively, the gaps can be repaired using random oligonucleotides of sufficient length to bring the gaps and a ligase.

Separating Hairpin Loops

As discussed above, the method of the invention may produce a plurality of modified double stranded polynucleotides whose strands are linked by a hairpin loop at one end. In such embodiments, the method preferably further comprises (d) separating the two strands of at least one modified double stranded polynucleotide to produce at least one single stranded polynucleotide comprising one strand of the modified double stranded polynucleotide linked to the other strand of the modified double stranded polynucleotide. The two strands may be separated in any way. The two strands are preferably separated by contacting the modified double stranded polynucleotide with a polynucleotide binding protein as discussed below.

Products of the Invention

The invention also provides a population of double stranded MuA substrates for modifying a template polynucleotide, wherein each substrate comprises at least one overhang of universal nucleotides. The invention also provides a population of double stranded MuA substrates for modifying a template polynucleotide, wherein each substrate comprises (i) at least one overhang and (ii) at least one nucleotide in the same strand as the at least one overhang which comprises a nucleoside that is not present in the template polynucleotide. The substrates may be any of those described above. The substrates preferably comprise a double stranded portion as defined above. The double stranded portion preferably comprises SEQ ID NOS: 24-25 as discussed above. The double stranded portion more preferably comprises SEQ ID NO: 24 and SEQ ID NO: 26 as discussed above. Preferred populations of the invention are those in which:

-   -   each substrate comprises an overhang at one end and a hairpin         loop at the other end;     -   each substrate is preferably a Y substrate with an overhang at         one end and a region that is not complementary at the other end;     -   a proportion of the substrates in the population comprise an         overhang at one end and a hairpin loop at the other end and a         proportion of the substrates in the population are Y substrates         with an overhang at one end and a region that is not         complementary at the other end;     -   a proportion of the substrates in the population comprise an         overhang at one end and a hairpin loop at the other end and a         proportion of the substrates in the population comprise an         overhang at one end and a leader sequence or coupling means at         the other end; and     -   a proportion of the substrates in the population comprise an         overhang at one end and a hairpin loop at the other end and a         proportion of the substrates in the population comprise an         overhang at one end and a leader sequence and a coupling means         at the other end.

The proportion values given above in connection the method of the invention are equally applicable to the populations of the invention.

The invention also provides a plurality of polynucleotides modified using the method of the invention. The plurality of polynucleotides may be in any of the forms discussed above. Preferred pluralities are those which comprise:

-   -   double stranded polynucleotides which are linked by a hairpin         loop at one or both ends;     -   double stranded polynucleotides which have a non-complementary         region at one or both ends;     -   double stranded polynucleotides which have a hairpin loop at one         end and a non-complementary region at the other end.     -   double stranded polynucleotides which have a hairpin loop at one         end and a leader sequence or coupling means at the other end;     -   double stranded polynucleotides which have a hairpin loop at one         end and a leader sequence and coupling means at the other end.

The population or plurality may be isolated, substantially isolated, purified or substantially purified. A population or plurality is isolated or purified if it is completely free of any other components, such as the template polynucleotide, lipids or pores. A population or plurality is substantially isolated if it is mixed with carriers or diluents which will not interfere with its intended use. For instance, a population or plurality is substantially isolated or substantially purified if it is present in a form that comprises less than 10%, less than 5%, less than 2% or less than 1% of other components, such as lipids or pores.

Characterisation Method of the Invention

The invention also provides a method of characterising at least one polynucleotide modified using a method of the invention. The method comprises (a) contacting the modified polynucleotide with a transmembrane pore such that at least one strand of the polynucleotide moves through the pore. The method also comprises (b) taking one or more measurements as the at least one strand moves with respect to the pore wherein the measurements are indicative of one or more characteristics of the at least one strand and thereby characterising the modified polynucleotide. If the modified polynucleotide comprises a hairpin loop, the method preferably comprises (a) contacting the modified polynucleotide with a transmembrane pore such that both strands of the polynucleotide move through the pore and (b) taking one or more measurements as both strands move with respect to the pore wherein the measurements are indicative of one or more characteristics of both strands and thereby characterising the modified polynucleotide.

This method is preferably carried out with a potential applied across the pore. The applied potential may be a voltage potential. Alternatively, the applied potential may be a chemical potential. An example of this is using a salt gradient across an amphiphilic layer. A salt gradient is disclosed in Holden et al., J Am Chem Soc. 2007 Jul. 11; 129(27):8650-5. In some instances, the current passing through the pore as the polynucleotide moves with respect to the pore is used to determine the sequence of the modified polynucleotide. This is strand sequencing.

The invention also provides a method of characterising a template polynucleotide. In step (a), the template polynucleotide is modified using the method of the invention. The method then comprises (b) contacting each modified polynucleotide with a transmembrane pore such that at least one strand of each polynucleotide moves through the pore. The method also comprises (c) taking one or more measurements as the at least one strand of each polynucleotide moves with respect to the pore wherein the measurements are indicative of one or more characteristics of the at least one strand of each polynucleotide and thereby characterising the template polynucleotide. If the modified polynucleotides comprise a hairpin loop, the method preferably comprises (b) contacting each modified polynucleotide with a transmembrane pore such that both strands of each polynucleotide move through the pore and (c) taking one or more measurements as both strands of each polynucleotide move with respect to the pore wherein the measurements are indicative of one or more characteristics of both strands of each polynucleotide and thereby characterising the template polynucleotide.

Steps (b) and (c) are preferably carried out with a potential applied across the pore as described above. In some instances, the current passing through the pore as the at least one strand of each polynucleotide moves with respect to the pore is used to determine the sequence of each modified polynucleotide. This is strand sequencing. The sequence of the template polynucleotide may then be reconstructed. In particular, the method preferably further comprises d) aligning the sequences of the plurality of modified polynucleotides to produce the sequence of the template polynucleotide and thereby sequencing the template polynucleotide. Suitable methods are disclosed in Baker, M., Nature Methods, 2012, 9, 4, 333-337 and Flicek, P. and Birney E., Nature Methods Supplement, 2009, 6, 11, S6-S12.

The whole or only part of the modified or template polynucleotide may be characterised, for instance sequenced, using this method. The length of the template polynucleotide is discussed above.

A transmembrane pore is a structure that crosses the membrane to some degree. It permits hydrated ions driven by an applied potential to flow across or within the membrane. The transmembrane pore typically crosses the entire membrane so that hydrated ions may flow from one side of the membrane to the other side of the membrane. However, the transmembrane pore does not have to cross the membrane. It may be closed at one end. For instance, the pore may be a well in the membrane along which or into which hydrated ions may flow.

Any transmembrane pore may be used in the invention. The pore may be biological or artificial. Suitable pores include, but are not limited to, protein pores, polynucleotide pores and solid state pores.

Any membrane may be used in accordance with the invention. Suitable membranes are well-known in the art. The membrane is preferably an amphiphilic layer. An amphiphilic layer is a layer formed from amphiphilic molecules, such as phospholipids, which have both at least one hydrophilic portion and at least one lipophilic or hydrophobic portion. The amphiphilic layer may be a monolayer or a bilayer. The amphiphilic molecules may be synthetic or naturally occurring. Non-naturally occurring amphiphiles and amphiphiles which form a monolayer are known in the art and include, for example, block copolymers (Gonzalez-Perez et al., Langmuir, 2009, 25, 10447-10450). Block copolymers are polymeric materials in which two or more monomer sub-units are polymerized together to create a single polymer chain. Block copolymers typically have properties that are contributed by each monomer sub-unit. However, a block copolymer may have unique properties that polymers formed from the individual sub-units do not possess. Block copolymers can be engineered such that one of the monomer sub-units is hydrophobic (i.e. lipophilic), whilst the other sub-unit(s) are hydrophilic whilst in aqueous media. In this case, the block copolymer may possess amphiphilic properties and may form a structure that mimics a biological membrane. The block copolymer may be a diblock (consisting of two monomer sub-units), but may also be constructed from more than two monomer sub-units to form more complex arrangements that behave as amphiphiles. The copolymer may be a triblock, tetrablock or pentablock copolymer.

The amphiphilic layer is typically a planar lipid bilayer or a supported bilayer.

The amphiphilic layer is typically a lipid bilayer. Lipid bilayers are models of cell membranes and serve as excellent platforms for a range of experimental studies. For example, lipid bilayers can be used for in vitro investigation of membrane proteins by single-channel recording. Alternatively, lipid bilayers can be used as biosensors to detect the presence of a range of substances. The lipid bilayer may be any lipid bilayer. Suitable lipid bilayers include, but are not limited to, a planar lipid bilayer, a supported bilayer or a liposome. The lipid bilayer is preferably a planar lipid bilayer. Suitable lipid bilayers are disclosed in International Application No. PCT/GB08/000563 (published as WO 2008/102121), International Application No. PCT/GB08/004127 (published as WO 2009/077734) and International Application No. PCT/GB2006/001057 (published as WO 2006/100484).

Methods for forming lipid bilayers are known in the art. Suitable methods are disclosed in the Example. Lipid bilayers are commonly formed by the method of Montal and Mueller (Proc. Natl. Acad. Sci. USA., 1972; 69: 3561-3566), in which a lipid monolayer is carried on aqueous solution/air interface past either side of an aperture which is perpendicular to that interface.

The method of Montal & Mueller is popular because it is a cost-effective and relatively straightforward method of forming good quality lipid bilayers that are suitable for protein pore insertion. Other common methods of bilayer formation include tip-dipping, painting bilayers and patch-clamping of liposome bilayers.

In a preferred embodiment, the lipid bilayer is formed as described in International Application No. PCT/GB08/004127 (published as WO 2009/077734).

In another preferred embodiment, the membrane is a solid state layer. A solid-state layer is not of biological origin. In other words, a solid state layer is not derived from or isolated from a biological environment such as an organism or cell, or a synthetically manufactured version of a biologically available structure. Solid state layers can be formed from both organic and inorganic materials including, but not limited to, microelectronic materials, insulating materials such as Si₃N₄, Al₂O₃, and SiO, organic and inorganic polymers such as polyamide, plastics such as Teflon® or elastomers such as two-component addition-cure silicone rubber, and glasses. The solid state layer may be formed from monatomic layers, such as graphene, or layers that are only a few atoms thick. Suitable graphene layers are disclosed in International Application No. PCT/US2008/010637 (published as WO 2009/035647).

The method is typically carried out using (i) an artificial amphiphilic layer comprising a pore, (ii) an isolated, naturally-occurring lipid bilayer comprising a pore, or (iii) a cell having a pore inserted therein. The method is typically carried out using an artificial amphiphilic layer, such as an artificial lipid bilayer. The layer may comprise other transmembrane and/or intramembrane proteins as well as other molecules in addition to the pore. Suitable apparatus and conditions are discussed below. The method of the invention is typically carried out in vitro.

The modified polynucleotide(s) may be coupled to the membrane. This may be done using any known method. If the membrane is an amphiphilic layer, such as a lipid bilayer (as discussed in detail above), the modified polynucleotide(s) are preferably coupled to the membrane via a polypeptide present in the membrane or a hydrophobic anchor present in the membrane. The hydrophobic anchor is preferably a lipid, fatty acid, sterol, carbon nanotube or amino acid.

The modified polynucleotide(s) may be coupled directly to the membrane. The modified polynucleotide(s) are preferably coupled to the membrane via a linker. Preferred linkers include, but are not limited to, polymers, such as polynucleotides, polyethylene glycols (PEGs) and polypeptides. If a modified polynucleotide is coupled directly to the membrane, then some data will be lost as the characterising run cannot continue to the end of the polynucleotide due to the distance between the membrane and the pore. If a linker is used, then the polynucleotide can be processed to completion. If a linker is used, the linker may be attached to the polynucleotide at any position. The linker is preferably attached to the polynucleotide at the tail polymer.

The coupling may be stable or transient. For certain applications, the transient nature of the coupling is preferred. If a stable coupling molecule were attached directly to either the 5′ or 3′ end of a polynucleotide, then some data will be lost as the characterising run cannot continue to the end of the polynucleotide due to the distance between the bilayer and the pore. If the coupling is transient, then when the coupled end randomly becomes free of the bilayer, then the polynucleotide can be processed to completion. Chemical groups that form stable or transient links with the membrane are discussed in more detail below. The modified polynucleotide(s) may be transiently coupled to an amphiphilic layer, such as a lipid bilayer using cholesterol or a fatty acyl chain. Any fatty acyl chain having a length of from 6 to 30 carbon atoms, such as hexadecanoic acid, may be used.

In preferred embodiments, the modified polynucleotide(s) are coupled to an amphiphilic layer. Coupling of polynucleotides to synthetic lipid bilayers has been carried out previously with various different tethering strategies. These are summarised in Table 1 below.

TABLE 1 Attach- ment Type of group coupling Reference Thiol Stable Yoshina-Ishii, C. and S. G. Boxer (2003). “Arrays of mobile tethered vesicles on supported lipid bilayers.” J Am Chem Soc 125(13): 3696-7. Biotin Stable Nikolov, V., R. Lipowsky, et al. (2007). “Behaviour of giant vesicles with anchored DNA molecules.” Biophys J 92(12): 4356-68 Choles- Transient Pfeiffer, I. and F. Hook (2004). “Bivalent terol cholesterol- based coupling of oligonucleotides to lipid membrane assemblies.” J Am Chem Soc 126(33): 10224-5 Lipid Stable van Lengerich, B., R. J. Rawle, et al. “Covalent attachment of lipid vesicles to a fluid-supported bilayer allows observation of DNA-mediated vesicle interactions.” Langmuir 26(11): 8666-72

Polynucleotides may be functionalized using a modified phosphoramidite in the synthesis reaction, which is easily compatible for the addition of reactive groups, such as thiol, cholesterol, lipid and biotin groups. These different attachment chemistries give a suite of attachment options for polynucleotides. Each different modification group tethers the polynucleotide in a slightly different way and coupling is not always permanent so giving different dwell times for the polynucleotide to the bilayer. The advantages of transient coupling are discussed above.

Coupling of polynucleotides can also be achieved by a number of other means provided that a reactive group can be added to the polynucleotide. The addition of reactive groups to either end of the DNA has been reported previously. A thiol group can be added to the 5′ of ssDNA using polynucleotide kinase and ATPγS (Grant, G. P. and P. Z. Qin (2007). “A facile method for attaching nitroxide spin labels at the 5′ terminus of nucleic acids.” Nucleic Acids Res 35(10): e77). A more diverse selection of chemical groups, such as biotin, thiols and fluorophores, can be added using terminal transferase to incorporate modified oligonucleotides to the 3′ of ssDNA (Kumar, A., P. Tchen, et al. (1988). “Nonradioactive labeling of synthetic oligonucleotide probes with terminal deoxynucleotidyl transferase.” Anal Biochem 169(2): 376-82).

Alternatively, the reactive group could be considered to be the addition of a short piece of DNA complementary to one already coupled to the bilayer, so that attachment can be achieved via hybridisation. Ligation of short pieces of ssDNA have been reported using T4 RNA ligase I (Troutt, A. B., M. G. McHeyzer-Williams, et al. (1992). “Ligation-anchored PCR: a simple amplification technique with single-sided specificity.” Proc Natl Acad Sci USA 89(20): 9823-5). Alternatively either ssDNA or dsDNA could be ligated to native dsDNA and then the two strands separated by thermal or chemical denaturation. To native dsDNA, it is possible to add either a piece of ssDNA to one or both of the ends of the duplex, or dsDNA to one or both ends. Then, when the duplex is melted, each single strand will have either a 5′ or 3′ modification if ssDNA was used for ligation or a modification at the 5′ end, the 3′ end or both if dsDNA was used for ligation. If the polynucleotide is a synthetic strand, the coupling chemistry can be incorporated during the chemical synthesis of the polynucleotide. For instance, the polynucleotide can be synthesized using a primer with a reactive group attached to it.

A common technique for the amplification of sections of genomic DNA is using polymerase chain reaction (PCR). Here, using two synthetic oligonucleotide primers, a number of copies of the same section of DNA can be generated, where for each copy the 5′ of each strand in the duplex will be a synthetic polynucleotide. By using an reverse primer that has a reactive group, such as a cholesterol, thiol, biotin or lipid, each copy of the amplified target DNA will contain a reactive group for coupling.

The transmembrane pore is preferably a transmembrane protein pore. A transmembrane protein pore is a polypeptide or a collection of polypeptides that permits hydrated ions, such as analyte, to flow from one side of a membrane to the other side of the membrane. In the present invention, the transmembrane protein pore is capable of forming a pore that permits hydrated ions driven by an applied potential to flow from one side of the membrane to the other. The transmembrane protein pore preferably permits analyte such as nucleotides to flow from one side of the membrane, such as a lipid bilayer, to the other. The transmembrane protein pore allows a polynucleotide, such as DNA or RNA, to be moved through the pore.

The transmembrane protein pore may be a monomer or an oligomer. The pore is preferably made up of several repeating subunits, such as 6, 7, 8 or 9 subunits. The pore is preferably a hexameric, heptameric, octameric or nonameric pore.

The transmembrane protein pore typically comprises a barrel or channel through which the ions may flow. The subunits of the pore typically surround a central axis and contribute strands to a transmembrane β barrel or channel or a transmembrane α-helix bundle or channel.

The barrel or channel of the transmembrane protein pore typically comprises amino acids that facilitate interaction with analyte, such as nucleotides, polynucleotides or nucleic acids. These amino acids are preferably located near a constriction of the barrel or channel. The transmembrane protein pore typically comprises one or more positively charged amino acids, such as arginine, lysine or histidine, or aromatic amino acids, such as tyrosine or tryptophan. These amino acids typically facilitate the interaction between the pore and nucleotides, polynucleotides or nucleic acids.

Transmembrane protein pores for use in accordance with the invention can be derived from β-barrel pores or α-helix bundle pores. β-barrel pores comprise a barrel or channel that is formed from β-strands. Suitable β-barrel pores include, but are not limited to, β-toxins, such as α-hemolysin, anthrax toxin and leukocidins, and outer membrane proteins/porins of bacteria, such as Mycobacterium smegmatis porin (Msp), for example MspA MspB, MspC or MspD, outer membrane porin F (OmpF), outer membrane porin G (OmpG), outer membrane phospholipase A and Neisseria autotransporter lipoprotein (NalP). α-helix bundle pores comprise a barrel or channel that is formed from α-helices. Suitable α-helix bundle pores include, but are not limited to, inner membrane proteins and α outer membrane proteins, such as WZA and ClyA toxin. The transmembrane pore may be derived from Msp or from α-hemolysin (α-HL).

The transmembrane protein pore is preferably derived from Msp, preferably from MspA. Such a pore will be oligomeric and typically comprises 7, 8, 9 or 10 monomers derived from Msp. The pore may be a homo-oligomeric pore derived from Msp comprising identical monomers. Alternatively, the pore may be a hetero-oligomeric pore derived from Msp comprising at least one monomer that differs from the others. Preferably the pore is derived from MspA or a homolog or paralog thereof.

A monomer derived from Msp typically comprises the sequence shown in SEQ ID NO: 2 or a variant thereof. SEQ ID NO: 2 is the MS-(B1)8 mutant of the MspA monomer. It includes the following mutations: D90N, D91N, D93N, D118R, D134R and E139K. A variant of SEQ ID NO: 2 is a polypeptide that has an amino acid sequence which varies from that of SEQ ID NO: 2 and which retains its ability to form a pore. The ability of a variant to form a pore can be assayed using any method known in the art. For instance, the variant may be inserted into an amphiphilic layer along with other appropriate subunits and its ability to oligomerise to form a pore may be determined. Methods are known in the art for inserting subunits into membranes, such as amphiphilic layers. For example, subunits may be suspended in a purified form in a solution containing a lipid bilayer such that it diffuses to the lipid bilayer and is inserted by binding to the lipid bilayer and assembling into a functional state. Alternatively, subunits may be directly inserted into the membrane using the “pick and place” method described in M. A. Holden, H. Bayley. J. Am. Chem. Soc. 2005, 127, 6502-6503 and International Application No. PCT/GB2006/001057 (published as WO 2006/100484).

Over the entire length of the amino acid sequence of SEQ ID NO: 2, a variant will preferably be at least 50% homologous to that sequence based on amino acid identity. More preferably, the variant may be at least 55%, at least 60%, at least 65%, at least 70%, at least 75%, at least 80%, at least 85%, at least 90% and more preferably at least 95%, 97% or 99% homologous based on amino acid identity to the amino acid sequence of SEQ ID NO: 2 over the entire sequence. There may be at least 80%, for example at least 85%, 90% or 95%, amino acid identity over a stretch of 100 or more, for example 125, 150, 175 or 200 or more, contiguous amino acids (“hard homology”).

Standard methods in the art may be used to determine homology. For example the UWGCG Package provides the BESTFIT program which can be used to calculate homology, for example used on its default settings (Devereux et al (1984) Nucleic Acids Research 12, p 387-395). The PILEUP and BLAST algorithms can be used to calculate homology or line up sequences (such as identifying equivalent residues or corresponding sequences (typically on their default settings)), for example as described in Altschul S. F. (1993) J Mol Evol 36:290-300; Altschul, S. F et al (1990) J Mol Biol 215:403-10. Software for performing BLAST analyses is publicly available through the National Center for Biotechnology Information (ncbi.nlm.nih.gov/).

SEQ ID NO: 2 is the MS-(B1)8 mutant of the MspA monomer. The variant may comprise any of the mutations in the MspB, C or D monomers compared with MspA. The mature forms of MspB, C and D are shown in SEQ ID NOS: 5 to 7. In particular, the variant may comprise the following substitution present in MspB: A138P. The variant may comprise one or more of the following substitutions present in MspC: A96G, N102E and A138P. The variant may comprise one or more of the following mutations present in MspD: Deletion of G1, L2V, E5Q, L8V, D13G, W21A, D22E, K47T, I49H, I68V, D91G, A96Q, N102D, S103T, V1041, S136K and G141A. The variant may comprise combinations of one or more of the mutations and substitutions from Msp B, C and D. The variant preferably comprises the mutation L88N. A variant of SEQ ID NO: 2 has the mutation L88N in addition to all the mutations of MS-B1 and is called MS-(B2)8. The pore used in the invention is preferably MS-(B2)8. A variant of SEQ ID NO: 2 has the mutations G75S/G77S/L88N/Q126R in addition to all the mutations of MS-B1 and is called MS-B2C. The pore used in the invention is preferably MS-(B2)8 or MS-(B2C)8.

Amino acid substitutions may be made to the amino acid sequence of SEQ ID NO: 2 in addition to those discussed above, for example up to 1, 2, 3, 4, 5, 10, 20 or 30 substitutions. Conservative substitutions replace amino acids with other amino acids of similar chemical structure, similar chemical properties or similar side-chain volume. The amino acids introduced may have similar polarity, hydrophilicity, hydrophobicity, basicity, acidity, neutrality or charge to the amino acids they replace. Alternatively, the conservative substitution may introduce another amino acid that is aromatic or aliphatic in the place of a pre-existing aromatic or aliphatic amino acid. Conservative amino acid changes are well-known in the art and may be selected in accordance with the properties of the 20 main amino acids as defined in Table 2 below. Where amino acids have similar polarity, this can also be determined by reference to the hydropathy scale for amino acid side chains in Table 3.

TABLE 2 Chemical properties of amino acids Ala aliphatic, hydrophobic, Met hydrophobic, neutral neutral Cys polar, hydrophobic, neutral Asn polar, hydrophilic, neutral Asp polar, hydrophilic, charged Pro hydrophobic, neutral (−) Glu polar, hydrophilic, charged Gln polar, hydrophilic, neutral (−) Phe aromatic, hydrophobic, Arg polar, hydrophilic, charged neutral (+) Gly aliphatic, neutral Ser polar, hydrophilic, neutral His aromatic, polar, hydrophilic, Thr polar, hydrophilic, neutral charged (+) Ile aliphatic, hydrophobic, Val aliphatic, hydrophobic, neutral neutral Lys polar, hydrophilic, charged(+) Trp aromatic, hydrophobic, neutral Leu aliphatic, hydrophobic, Tyr aromatic, polar, neutral hydrophobic

TABLE 3 Hydropathy scale Side Chain Hydropath Ile 4.5 Val 4.2 Leu 3.8 Phe 2.8 Cys 2.5 Met 1.9 Ala 1.8 Gly −0.4 Thr −0.7 Ser −0.8 Trp −0.9 Tyr −1.3 Pro −1.6 His −3.2 Glu −3.5 Gln −3.5 Asp −3.5 Asn −3.5 Lys −3.9 Arg −4.5

One or more amino acid residues of the amino acid sequence of SEQ ID NO: 2 may additionally be deleted from the polypeptides described above. Up to 1, 2, 3, 4, 5, 10, 20 or 30 residues may be deleted, or more.

Variants may include fragments of SEQ ID NO: 2. Such fragments retain pore forming activity. Fragments may be at least 50, 100, 150 or 200 amino acids in length. Such fragments may be used to produce the pores. A fragment preferably comprises the pore forming domain of SEQ ID NO: 2. Fragments must include one of residues 88, 90, 91, 105, 118 and 134 of SEQ ID NO: 2. Typically, fragments include all of residues 88, 90, 91, 105, 118 and 134 of SEQ ID NO: 2.

One or more amino acids may be alternatively or additionally added to the polypeptides described above. An extension may be provided at the amino terminal or carboxy terminal of the amino acid sequence of SEQ ID NO: 2 or polypeptide variant or fragment thereof. The extension may be quite short, for example from 1 to 10 amino acids in length. Alternatively, the extension may be longer, for example up to 50 or 100 amino acids. A carrier protein may be fused to an amino acid sequence according to the invention. Other fusion proteins are discussed in more detail below.

As discussed above, a variant is a polypeptide that has an amino acid sequence which varies from that of SEQ ID NO: 2 and which retains its ability to form a pore. A variant typically contains the regions of SEQ ID NO: 2 that are responsible for pore formation. The pore forming ability of Msp, which contains a β-barrel, is provided by β-sheets in each subunit. A variant of SEQ ID NO: 2 typically comprises the regions in SEQ ID NO: 2 that form β-sheets. One or more modifications can be made to the regions of SEQ ID NO: 2 that form β-sheets as long as the resulting variant retains its ability to form a pore. A variant of SEQ ID NO: 2 preferably includes one or more modifications, such as substitutions, additions or deletions, within its α-helices and/or loop regions.

The monomers derived from Msp may be modified to assist their identification or purification, for example by the addition of histidine residues (a hist tag), aspartic acid residues (an asp tag), a streptavidin tag or a flag tag, or by the addition of a signal sequence to promote their secretion from a cell where the polypeptide does not naturally contain such a sequence. An alternative to introducing a genetic tag is to chemically react a tag onto a native or engineered position on the pore. An example of this would be to react a gel-shift reagent to a cysteine engineered on the outside of the pore. This has been demonstrated as a method for separating hemolysin hetero-oligomers (Chem Biol. 1997 July; 4(7):497-505).

The monomer derived from Msp may be labelled with a revealing label. The revealing label may be any suitable label which allows the pore to be detected. Suitable labels are described below.

The monomer derived from Msp may also be produced using D-amino acids. For instance, the monomer derived from Msp may comprise a mixture of L-amino acids and D-amino acids. This is conventional in the art for producing such proteins or peptides.

The monomer derived from Msp contains one or more specific modifications to facilitate nucleotide discrimination. The monomer derived from Msp may also contain other non-specific modifications as long as they do not interfere with pore formation. A number of non-specific side chain modifications are known in the art and may be made to the side chains of the monomer derived from Msp. Such modifications include, for example, reductive alkylation of amino acids by reaction with an aldehyde followed by reduction with NaBH₄, amidination with methylacetimidate or acylation with acetic anhydride.

The monomer derived from Msp can be produced using standard methods known in the art. The monomer derived from Msp may be made synthetically or by recombinant means. For example, the pore may be synthesized by in vitro translation and transcription (IVTT). Suitable methods for producing pores are discussed in International Application Nos. PCT/GB09/001690 (published as WO 2010/004273), PCT/GB09/001679 (published as WO 2010/004265) or PCT/GB10/000133 (published as WO 2010/086603). Methods for inserting pores into membranes are discussed.

The transmembrane protein pore is also preferably derived from α-hemolysin (α-HL). The wild type α-HL pore is formed of seven identical monomers or subunits (i.e. it is heptameric). The sequence of one monomer or subunit of α-hemolysin-NN is shown in SEQ ID NO: 4. The transmembrane protein pore preferably comprises seven monomers each comprising the sequence shown in SEQ ID NO: 4 or a variant thereof. Amino acids 1, 7 to 21, 31 to 34, 45 to 51, 63 to 66, 72, 92 to 97, 104 to 111, 124 to 136, 149 to 153, 160 to 164, 173 to 206, 210 to 213, 217, 218, 223 to 228, 236 to 242, 262 to 265, 272 to 274, 287 to 290 and 294 of SEQ ID NO: 4 form loop regions. Residues 113 and 147 of SEQ ID NO: 4 form part of a constriction of the barrel or channel of α-HL.

In such embodiments, a pore comprising seven proteins or monomers each comprising the sequence shown in SEQ ID NO: 4 or a variant thereof are preferably used in the method of the invention. The seven proteins may be the same (homo-heptamer) or different (hetero-heptamer).

A variant of SEQ ID NO: 4 is a protein that has an amino acid sequence which varies from that of SEQ ID NO: 4 and which retains its pore forming ability. The ability of a variant to form a pore can be assayed using any method known in the art. For instance, the variant may be inserted into an amphiphilic layer, such as a lipid bilayer, along with other appropriate subunits and its ability to oligomerise to form a pore may be determined. Methods are known in the art for inserting subunits into amphiphilic layers, such as lipid bilayers. Suitable methods are discussed above.

The variant may include modifications that facilitate covalent attachment to or interaction with the construct. The variant preferably comprises one or more reactive cysteine residues that facilitate attachment to the construct. For instance, the variant may include a cysteine at one or more of positions 8, 9, 17, 18, 19, 44, 45, 50, 51, 237, 239 and 287 and/or on the amino or carboxy terminus of SEQ ID NO: 4. Preferred variants comprise a substitution of the residue at position 8, 9, 17, 237, 239 and 287 of SEQ ID NO: 4 with cysteine (ABC, T9C, N17C, K237C, S239C or E287C). The variant is preferably any one of the variants described in International Application No. PCT/GB09/001690 (published as WO 2010/004273), PCT/GB09/001679 (published as WO 2010/004265) or PCT/GB10/000133 (published as WO 2010/086603).

The variant may also include modifications that facilitate any interaction with nucleotides.

The variant may be a naturally occurring variant which is expressed naturally by an organism, for instance by a Staphylococcus bacterium. Alternatively, the variant may be expressed in vitro or recombinantly by a bacterium such as Escherichia coli. Variants also include non-naturally occurring variants produced by recombinant technology. Over the entire length of the amino acid sequence of SEQ ID NO: 4, a variant will preferably be at least 50% homologous to that sequence based on amino acid identity. More preferably, the variant polypeptide may be at least 55%, at least 60%, at least 65%, at least 70%, at least 75%, at least 80%, at least 85%, at least 90% and more preferably at least 95%, 97% or 99% homologous based on amino acid identity to the amino acid sequence of SEQ ID NO: 4 over the entire sequence. There may be at least 80%, for example at least 85%, 90% or 95%, amino acid identity over a stretch of 200 or more, for example 230, 250, 270 or 280 or more, contiguous amino acids (“hard homology”). Homology can be determined as discussed above.

Amino acid substitutions may be made to the amino acid sequence of SEQ ID NO: 4 in addition to those discussed above, for example up to 1, 2, 3, 4, 5, 10, 20 or 30 substitutions. Conservative substitutions may be made as discussed above.

One or more amino acid residues of the amino acid sequence of SEQ ID NO: 4 may additionally be deleted from the polypeptides described above. Up to 1, 2, 3, 4, 5, 10, 20 or 30 residues may be deleted, or more.

Variants may be fragments of SEQ ID NO: 4. Such fragments retain pore-forming activity. Fragments may be at least 50, 100, 200 or 250 amino acids in length. A fragment preferably comprises the pore-forming domain of SEQ ID NO: 4. Fragments typically include residues 119, 121, 135. 113 and 139 of SEQ ID NO: 4.

One or more amino acids may be alternatively or additionally added to the polypeptides described above. An extension may be provided at the amino terminus or carboxy terminus of the amino acid sequence of SEQ ID NO: 4 or a variant or fragment thereof. The extension may be quite short, for example from 1 to 10 amino acids in length. Alternatively, the extension may be longer, for example up to 50 or 100 amino acids. A carrier protein may be fused to a pore or variant.

As discussed above, a variant of SEQ ID NO: 4 is a subunit that has an amino acid sequence which varies from that of SEQ ID NO: 4 and which retains its ability to form a pore. A variant typically contains the regions of SEQ ID NO: 4 that are responsible for pore formation. The pore forming ability of α-HL, which contains a β-barrel, is provided by β-strands in each subunit. A variant of SEQ ID NO: 4 typically comprises the regions in SEQ ID NO: 4 that form β-strands. The amino acids of SEQ ID NO: 4 that form β-strands are discussed above. One or more modifications can be made to the regions of SEQ ID NO: 4 that form β-strands as long as the resulting variant retains its ability to form a pore. Specific modifications that can be made to the β-strand regions of SEQ ID NO: 4 are discussed above.

A variant of SEQ ID NO: 4 preferably includes one or more modifications, such as substitutions, additions or deletions, within its α-helices and/or loop regions. Amino acids that form α-helices and loops are discussed above.

The variant may be modified to assist its identification or purification as discussed above.

Pores derived from α-HL can be made as discussed above with reference to pores derived from Msp.

In some embodiments, the transmembrane protein pore is chemically modified. The pore can be chemically modified in any way and at any site. The transmembrane protein pore is preferably chemically modified by attachment of a molecule to one or more cysteines (cysteine linkage), attachment of a molecule to one or more lysines, attachment of a molecule to one or more non-natural amino acids, enzyme modification of an epitope or modification of a terminus. Suitable methods for carrying out such modifications are well-known in the art. The transmembrane protein pore may be chemically modified by the attachment of any molecule. For instance, the pore may be chemically modified by attachment of a dye or a fluorophore.

Any number of the monomers in the pore may be chemically modified. One or more, such as 2, 3, 4, 5, 6, 7, 8, 9 or 10, of the monomers is preferably chemically modified as discussed above.

The reactivity of cysteine residues may be enhanced by modification of the adjacent residues. For instance, the basic groups of flanking arginine, histidine or lysine residues will change the pKa of the cysteines thiol group to that of the more reactive S⁻ group. The reactivity of cysteine residues may be protected by thiol protective groups such as dTNB. These may be reacted with one or more cysteine residues of the pore before a linker is attached.

The molecule (with which the pore is chemically modified) may be attached directly to the pore or attached via a linker as disclosed in International Application No. PCT/GB09/001690 (published as WO 2010/004273), PCT/GB09/001679 (published as WO 2010/004265), PCT/GB10/000133 (published as WO 2010/086603) or PCT/GB10/000132 (published as WO 2010/086602).

Any of the proteins described herein, such as the transmembrane protein pores, may be modified to assist their identification or purification, for example by the addition of histidine residues (a his tag), aspartic acid residues (an asp tag), a streptavidin tag, a flag tag, a SUMO tag, a GST tag or a MBP tag, or by the addition of a signal sequence to promote their secretion from a cell where the polypeptide does not naturally contain such a sequence. An alternative to introducing a genetic tag is to chemically react a tag onto a native or engineered position on the pore or construct. An example of this would be to react a gel-shift reagent to a cysteine engineered on the outside of the pore. This has been demonstrated as a method for separating hemolysin hetero-oligomers (Chem Biol. 1997 July; 4(7):497-505).

The pore may be labelled with a revealing label. The revealing label may be any suitable label which allows the pore to be detected. Suitable labels include, but are not limited to, fluorescent molecules, radioisotopes, e.g. ¹²⁵I, ³⁵S, enzymes, antibodies, antigens, polynucleotides and ligands such as biotin.

Any of the proteins described herein, such as the transmembrane protein pores, may be made synthetically or by recombinant means. For example, the pore may be synthesized by in vitro translation and transcription (IVTT). The amino acid sequence of the pore may be modified to include non-naturally occurring amino acids or to increase the stability of the protein. When a protein is produced by synthetic means, such amino acids may be introduced during production. The pore may also be altered following either synthetic or recombinant production.

The pore may also be produced using D-amino acids. For instance, the pore or construct may comprise a mixture of L-amino acids and D-amino acids. This is conventional in the art for producing such proteins or peptides.

The pore may also contain other non-specific modifications as long as they do not interfere with pore formation or construct function. A number of non-specific side chain modifications are known in the art and may be made to the side chains of the protein(s). Such modifications include, for example, reductive alkylation of amino acids by reaction with an aldehyde followed by reduction with NaBH₄, amidination with methylacetimidate or acylation with acetic anhydride.

Any of the proteins described herein, such as the transmembrane protein pores, can be produced using standard methods known in the art. Polynucleotide sequences encoding a pore or construct may be derived and replicated using standard methods in the art. Polynucleotide sequences encoding a pore or construct may be expressed in a bacterial host cell using standard techniques in the art. The pore may be produced in a cell by in situ expression of the polypeptide from a recombinant expression vector. The expression vector optionally carries an inducible promoter to control the expression of the polypeptide. These methods are described in Sambrook, J. and Russell, D. (2001). Molecular Cloning: A Laboratory Manual, 3rd Edition. Cold Spring Harbor Laboratory Press, Cold Spring Harbor, N.Y.

The pore may be produced in large scale following purification by any protein liquid chromatography system from protein producing organisms or after recombinant expression. Typical protein liquid chromatography systems include FPLC, AKTA systems, the Bio-Cad system, the Bio-Rad BioLogic system and the Gilson HPLC system.

The method of the invention involves measuring one or more characteristics of the modified polynucleotide(s) or template polynucleotide. The method may involve measuring two, three, four or five or more characteristics of the polynucleotide. The one or more characteristics are preferably selected from (i) the length of the polynucleotide, (ii) the identity of the polynucleotide, (iii) the sequence of the polynucleotide, (iv) the secondary structure of the polynucleotide and (v) whether or not the polynucleotide is modified. Any combination of (i) to (v) may be measured in accordance with the invention.

For (i), the length of the polynucleotide may be measured for example by determining the number of interactions between the polynucleotide and the pore or the duration of interaction between the polynucleotide and the pore.

For (ii), the identity of the polynucleotide may be measured in a number of ways. The identity of the polynucleotide may be measured in conjunction with measurement of the sequence of the polynucleotide or without measurement of the sequence of the polynucleotide. The former is straightforward; the polynucleotide is sequenced and thereby identified. The latter may be done in several ways. For instance, the presence of a particular motif in the polynucleotide may be measured (without measuring the remaining sequence of the polynucleotide). Alternatively, the measurement of a particular electrical and/or optical signal in the method may identify the polynucleotide as coming from a particular source.

For (iii), the sequence of the polynucleotide can be determined as described previously. Suitable sequencing methods, particularly those using electrical measurements, are described in Stoddart D et al., Proc Natl Acad Sci, 12; 106(19):7702-7, Lieberman K R et al, J Am Chem Soc. 2010; 132(50):17961-72, and International Application WO 2000/28312.

For (iv), the secondary structure may be measured in a variety of ways. For instance, if the method involves an electrical measurement, the secondary structure may be measured using a change in dwell time or a change in current flowing through the pore. This allows regions of single-stranded and double-stranded polynucleotide to be distinguished.

For (v), the presence or absence of any modification may be measured. The method preferably comprises determining whether or not the polynucleotide is modified by methylation, by oxidation, by damage, with one or more proteins or with one or more labels, tags or spacers. Specific modifications will result in specific interactions with the pore which can be measured using the methods described below. For instance, methylcytosine may be distinguished from cytosine on the basis of the current flowing through the pore during its interaction with each nucleotide.

A variety of different types of measurements may be made. This includes without limitation: electrical measurements and optical measurements. Possible electrical measurements include: current measurements, impedance measurements, tunnelling measurements (Ivanov A P et al., Nano Lett. 2011 Jan. 12; 11(1):279-85), and FET measurements (International Application WO 2005/124888). Optical measurements may be combined with electrical measurements (Soni G V et al., Rev Sci Instrum. 2010 January; 81(1):014301). The measurement may be a transmembrane current measurement such as measurement of ionic current flowing through the pore.

Electrical measurements may be made using standard single channel recording equipment as describe in Stoddart D et al., Proc Natl Acad Sci, 12; 106(19):7702-7, Lieberman K R et al, J Am Chem Soc. 2010; 132(50):17961-72, and International Application WO-2000/28312. Alternatively, electrical measurements may be made using a multi-channel system, for example as described in International Application WO-2009/077734 and International Application WO-2011/067559.

In a preferred embodiment, the method comprises:

(a) contacting the/each modified polynucleotide with a transmembrane pore such that at least one strand of the/each polynucleotide moves through the pore; and

(b) measuring the current passing through the pore as at least one strand of the/each polynucleotide moves with respect to the pore wherein the current is indicative of one or more characteristics of the at least one strand of the/each polynucleotide and thereby characterising the modified/template polynucleotide.

The methods may be carried out using any apparatus that is suitable for investigating a membrane/pore system in which a pore is present in a membrane. The method may be carried out using any apparatus that is suitable for transmembrane pore sensing. For example, the apparatus comprises a chamber comprising an aqueous solution and a barrier that separates the chamber into two sections. The barrier typically has an aperture in which the membrane containing the pore is formed. Alternatively the barrier forms the membrane in which the pore is present.

The methods may be carried out using the apparatus described in International Application No. PCT/GB08/000562 (WO 2008/102120).

The methods may involve measuring the current passing through the pore as the/each polynucleotide moves with respect to the pore. Therefore the apparatus may also comprise an electrical circuit capable of applying a potential and measuring an electrical signal across the membrane and pore. The methods may be carried out using a patch clamp or a voltage clamp. The methods preferably involve the use of a voltage clamp.

Suitable conditions for measuring ionic currents through transmembrane protein pores are known in the art and disclosed in the Example. The method is typically carried out with a voltage applied across the membrane and pore. The voltage used is typically from +2 V to −2 V, typically −400 mV to +400 mV. The voltage used is preferably in a range having a lower limit selected from −400 mV, −300 mV, −200 mV, −150 mV, −100 mV, −50 mV, −20 mV and 0 mV and an upper limit independently selected from +10 mV, +20 mV, +50 mV, +100 mV, +150 mV, +200 mV, +300 mV and +400 mV. The voltage used is more preferably in the range 100 mV to 240 mV and most preferably in the range of 120 mV to 220 mV. It is possible to increase discrimination between different nucleotides by a pore by using an increased applied potential.

The methods are typically carried out in the presence of any charge carriers, such as metal salts, for example alkali metal salt, halide salts, for example chloride salts, such as alkali metal chloride salt. Charge carriers may include ionic liquids or organic salts, for example tetramethyl ammonium chloride, trimethylphenyl ammonium chloride, phenyltrimethyl ammonium chloride, or 1-ethyl-3-methyl imidazolium chloride. In the exemplary apparatus discussed above, the salt is present in the aqueous solution in the chamber. Potassium chloride (KCl), sodium chloride (NaCl), caesium chloride (CsCl) or a mixture of potassium ferrocyanide and potassium ferricyanide is typically used. KCl, NaCl and a mixture of potassium ferrocyanide and potassium ferricyanide are preferred. The salt concentration may be at saturation. The salt concentration may be 3 M or lower and is typically from 0.1 to 2.5 M, from 0.3 to 1.9 M, from 0.5 to 1.8 M, from 0.7 to 1.7 M, from 0.9 to 1.6 M or from 1 M to 1.4 M. The salt concentration is preferably from 150 mM to 1 M. The method is preferably carried out using a salt concentration of at least 0.3 M, such as at least 0.4 M, at least 0.5 M, at least 0.6 M, at least 0.8 M, at least 1.0 M, at least 1.5 M, at least 2.0 M, at least 2.5 M or at least 3.0 M. High salt concentrations provide a high signal to noise ratio and allow for currents indicative of the presence of a nucleotide to be identified against the background of normal current fluctuations.

The methods are typically carried out in the presence of a buffer. In the exemplary apparatus discussed above, the buffer is present in the aqueous solution in the chamber. Any buffer may be used in the method of the invention. Typically, the buffer is HEPES. Another suitable buffer is Tris-HCl buffer. The methods are typically carried out at a pH of from 4.0 to 12.0, from 4.5 to 10.0, from 5.0 to 9.0, from 5.5 to 8.8, from 6.0 to 8.7 or from 7.0 to 8.8 or 7.5 to 8.5. The pH used is preferably about 7.5.

The methods may be carried out at from 0° C. to 100° C., from 15° C. to 95° C., from 16° C. to 90° C., from 17° C. to 85° C., from 18° C. to 80° C., 19° C. to 70° C., or from 20° C. to 60° C. The methods are typically carried out at room temperature. The methods are optionally carried out at a temperature that supports enzyme function, such as about 37° C.

The method preferably further comprises contacting the/each modified polynucleotide with a polynucleotide binding protein such that the protein controls the movement of the at least one strand of the/each polynucleotide through the pore. If the modified polynucleotide(s) comprise a hairpin loop, the method preferably further comprises contacting the/each modified polynucleotide with a polynucleotide binding protein such that the protein controls the movement of both strands of the/each polynucleotide through the pore.

More preferably, the method comprises (a) contacting the/each modified polynucleotide with a transmembrane pore and a polynucleotide binding protein such that at least one strand of the/each polynucleotide moves through the pore and the protein controls the movement of the at least one strand of the/each polynucleotide through the pore and (b) measuring the current passing through the pore as the at least one strand of the/each polynucleotide moves with respect to the pore wherein the current is indicative of one or more characteristics of the at least one strand of the/each polynucleotide and thereby characterising the modified/template polynucleotide. If the modified polynucleotide(s) comprise a hairpin loop, the method more preferable comprises (a) contacting the/each modified polynucleotide with a transmembrane pore and a polynucleotide binding protein such that both strands of the/each polynucleotide moves through the pore and the protein controls the movement of both strands of the/each polynucleotide through the pore and (b) measuring the current passing through the pore as both strands of the/each polynucleotide moves with respect to the pore wherein the current is indicative of one or more characteristics of both strands of the/each polynucleotide and thereby characterising the modified/template polynucleotide.

The polynucleotide binding protein may be any protein that is capable of binding to the polynucleotide and controlling its movement through the pore. It is straightforward in the art to determine whether or not a protein binds to a polynucleotide. The protein typically interacts with and modifies at least one property of the polynucleotide. The protein may modify the polynucleotide by cleaving it to form individual nucleotides or shorter chains of nucleotides, such as di- or trinucleotides. The moiety may modify the polynucleotide by orienting it or moving it to a specific position, i.e. controlling its movement.

The polynucleotide binding protein is preferably a polynucleotide handling enzyme. A polynucleotide handling enzyme is a polypeptide that is capable of interacting with and modifying at least one property of a polynucleotide. The enzyme may modify the polynucleotide by cleaving it to form individual nucleotides or shorter chains of nucleotides, such as di- or trinucleotides. The enzyme may modify the polynucleotide by orienting it or moving it to a specific position. The polynucleotide handling enzyme does not need to display enzymatic activity as long as it is capable of binding the polynucleotide and controlling its movement through the pore. For instance, the enzyme may be modified to remove its enzymatic activity or may be used under conditions which prevent it from acting as an enzyme. Such conditions are discussed in more detail below.

The polynucleotide handling enzyme is preferably derived from a nucleolytic enzyme. The polynucleotide handling enzyme used in the construct of the enzyme is more preferably derived from a member of any of the Enzyme Classification (EC) groups 3.1.11, 3.1.13, 3.1.14, 3.1.15, 3.1.16, 3.1.21, 3.1.22, 3.1.25, 3.1.26, 3.1.27, 3.1.30 and 3.1.31. The enzyme may be any of those disclosed in International Application No. PCT/GB10/000133 (published as WO 2010/086603).

Preferred enzymes are polymerases, exonucleases, helicases and topoisomerases, such as gyrases. Suitable enzymes include, but are not limited to, exonuclease I from E. coli (SEQ ID NO: 11), exonuclease III enzyme from E. coli (SEQ ID NO: 13), RecJ from T. thermophilus (SEQ ID NO: 15) and bacteriophage lambda exonuclease (SEQ ID NO: 17) and variants thereof. Three subunits comprising the sequence shown in SEQ ID NO: 15 or a variant thereof interact to form a trimer exonuclease. The enzyme is preferably Phi29 DNA polymerase (SEQ ID NO: 9) or a variant thereof. The topoisomerase is preferably a member of any of the Moiety Classification (EC) groups 5.99.1.2 and 5.99.1.3.

The enzyme is most preferably derived from a helicase, such as Hel308 Mbu (SEQ ID NO: 18), Hel308 Csy (SEQ ID NO: 19), Hel308 Mhu (SEQ ID NO: 20), TraI Eco (SEQ ID NO: 21), XPD Mbu (SEQ ID NO: 22) or a variant thereof.

A variant of SEQ ID NOS: 9, 11, 13, 15, 17, 18, 19, 20, 21, 22 or 23 is an enzyme that has an amino acid sequence which varies from that of SEQ ID NOS: 9, 11, 13, 15, 17, 18, 19, 20, 21, 22 or 23 and which retains polynucleotide binding ability. This can be measured using any method known in the art. For instance, the variant can be contacted with a polynucleotide and its ability to bind to and move along the polynucleotide can be measured. The variant may include modifications that facilitate binding of the polynucleotide and/or facilitate its activity at high salt concentrations and/or room temperature.

Over the entire length of the amino acid sequence of SEQ ID NOS: 9, 11, 13, 15, 17, 18, 19, 20, 21, 22 or 23, a variant will preferably be at least 50% homologous to that sequence based on amino acid identity. More preferably, the variant polypeptide may be at least 55%, at least 60%, at least 65%, at least 70%, at least 75%, at least 80%, at least 85%, at least 90% and more preferably at least 95%, 97% or 99% homologous based on amino acid identity to the amino acid sequence of SEQ ID NOS: 9, 11, 13, 15, 17, 18, 19, 20, 21, 22 or 23 over the entire sequence. There may be at least 80%, for example at least 85%, 90% or 95%, amino acid identity over a stretch of 200 or more, for example 230, 250, 270, 280, 300, 400, 500, 600, 700, 800, 900 or 1000 or more, contiguous amino acids (“hard homology”). Homology is determined as described above. The variant may differ from the wild-type sequence in any of the ways discussed above with reference to SEQ ID NO: 2 and SEQ ID NO: 4 above. The enzyme may be covalently attached to the pore. Any method may be used to covalently attach the enzyme to the pore.

In strand sequencing, the polynucleotide is translocated through the pore either with or against an applied potential. Exonucleases that act progressively or processively on double stranded polynucleotides can be used on the cis side of the pore to feed the remaining single strand through under an applied potential or the trans side under a reverse potential. Likewise, a helicase that unwinds the double stranded DNA can also be used in a similar manner. A polymerase may also be used. There are also possibilities for sequencing applications that require strand translocation against an applied potential, but the DNA must be first “caught” by the enzyme under a reverse or no potential. With the potential then switched back following binding the strand will pass cis to trans through the pore and be held in an extended conformation by the current flow. The single strand DNA exonucleases or single strand DNA dependent polymerases can act as molecular motors to pull the recently translocated single strand back through the pore in a controlled stepwise manner, trans to cis, against the applied potential.

The method of characterising a modified or a template polynucleotide preferably involves contacting the polynucleotide with a pore and a polynucleotide binding protein derived from a helicase. Any helicase may be used in the method. Helicases may work in two modes with respect to the pore. First, the method is preferably carried out using a helicase such that it moves the polynucleotide through the pore with the field resulting from the applied voltage. In this mode the 5′ end of the polynucleotide is first captured in the pore, and the helicase moves the polynucleotide into the pore such that it is passed through the pore with the field until it finally translocates through to the trans side of the bilayer. Alternatively, the method is preferably carried out such that a helicase moves the polynucleotide through the pore against the field resulting from the applied voltage. In this mode the 3′ end of the polynucleotide is first captured in the pore, and the helicase moves the polynucleotide through the pore such that it is pulled out of the pore against the applied field until finally ejected back to the cis side of the bilayer.

The polynucleotide may be contacted with the polynucleotide binding protein and the pore in any order. It is preferred that, when the polynucleotide is contacted with the polynucleotide binding protein, such as a helicase, and the pore, the polynucleotide firstly forms a complex with the protein. When the voltage is applied across the pore, the polynucleotide/protein complex then forms a complex with the pore and controls the movement of the polynucleotide through the pore.

The method is typically carried out in the presence of free nucleotides or free nucleotide analogues and an enzyme cofactor that facilitates the action of the polynucleotide binding protein. The free nucleotides may be one or more of any of the individual nucleotides discussed above. The free nucleotides include, but are not limited to, adenosine monophosphate (AMP), adenosine diphosphate (ADP), adenosine triphosphate (ATP), guanosine monophosphate (GMP), guanosine diphosphate (GDP), guanosine triphosphate (GTP), thymidine monophosphate (TMP), thymidine diphosphate (TDP), thymidine triphosphate (TTP), uridine monophosphate (UMP), uridine diphosphate (UDP), uridine triphosphate (UTP), cytidine monophosphate (CMP), cytidine diphosphate (CDP), cytidine triphosphate (CTP), cyclic adenosine monophosphate (cAMP), cyclic guanosine monophosphate (cGMP), deoxyadenosine monophosphate (dAMP), deoxyadenosine diphosphate (dADP), deoxyadenosine triphosphate (dATP), deoxyguanosine monophosphate (dGMP), deoxyguanosine diphosphate (dGDP), deoxyguanosine triphosphate (dGTP), deoxythymidine monophosphate (dTMP), deoxythymidine diphosphate (dTDP), deoxythymidine triphosphate (dTTP), deoxyuridine monophosphate (dUMP), deoxyuridine diphosphate (dUDP), deoxyuridine triphosphate (dUTP), deoxycytidine monophosphate (dCMP), deoxycytidine diphosphate (dCDP) and deoxycytidine triphosphate (dCTP). The free nucleotides are preferably selected from AMP, TMP, GMP, CMP, UMP, dAMP, dTMP, dGMP or dCMP. The free nucleotides are preferably adenosine triphosphate (ATP). The enzyme cofactor is a factor that allows the construct to function. The enzyme cofactor is preferably a divalent metal cation. The divalent metal cation is preferably Mg²⁺, Mn²⁺, Ca²⁺ or Co²⁺. The enzyme cofactor is most preferably Mg²⁺.

Kits

The present invention also provides a kit for modifying a template polynucleotide. The kit comprises (a) a population of MuA substrates of the invention and (b) a MuA transposase. Any of the embodiments discussed above with reference to the methods and products of the invention equally apply to the kits.

The kit may further comprise the components of a membrane, such as the components of an amphiphilic layer or a lipid bilayer. The kit may further comprise the components of a transmembrane pore. The kit may further comprise a polynucleotide binding protein. Suitable membranes, pores and polynucleotide binding proteins are discussed above.

The kit of the invention may additionally comprise one or more other reagents or instruments which enable any of the embodiments mentioned above to be carried out. Such reagents or instruments include one or more of the following: suitable buffer(s) (aqueous solutions), means to obtain a sample from a subject (such as a vessel or an instrument comprising a needle), means to amplify and/or express polynucleotides, a membrane as defined above or voltage or patch clamp apparatus. Reagents may be present in the kit in a dry state such that a fluid sample resuspends the reagents. The kit may also, optionally, comprise instructions to enable the kit to be used in the method of the invention or details regarding which patients the method may be used for. The kit may, optionally, comprise nucleotides.

The following Example illustrates the invention.

Example 1

This example shows that the MuA transposase was able to insert MuA substrates which contained at least one nucleoside which was not present in the template polynucleotide into a template double-stranded polynucleotide. In this example the one nucleotide which was not present in the template was a dUMP.

Materials and Methods

1.1 Anneal of DNA Strands to Form the MuA Substrates

MuA substrate 1 and 2 were prepared as shown in Table 4 below. The construct A and B sample mixtures were then heated to 95° C. for 2 minutes and then cooled to 16° C. at a rate of 2° C. per minute. This allowed SEQ ID NO: 27 which was attached to SEQ ID NO: 24 by four C3 spacer units to anneal to either SEQ ID NO: 28 (MuA substrate 1) or SEQ ID NO: 26 (MuA substrate 2). The DNA substrate designs of the two MuA substrates formed are shown in FIG. 15.

TABLE 4 Final MuA MuA Concen- Reagent substrate 1 substrate 2 trations Water 12 uL 12 uL 0.25M NaCl, 4 uL 4 uL 50 mM NaCl, 50 mM Tris pH 7.5 10 mM Tris SEQ ID NO: 27 attached to 2 uL 2 uL 10 uM SEQ ID NO: 24 by four C3 spacer units (100 uM) SEQ ID NO: 28 (100 uM) 2 uL 10 uM SEQ ID NO: 26 (100 uM) 2 uL 10 uM Total 20 uL 20 uL 1.2 Fragmentation of the DNA Template Using the MuA Transposase

Samples 1-5 were prepared as shown in Table 5 below. The samples were then incubated at 30° C. for 1 hour and heat inactivated at 75° C. for 10 minutes. An aliquot (1 μL) of each of samples 1-5 was run on a 12000 Agilent chip and the remainder on a 0.8% TAE agarose gel (60 minutes at 100 V, see FIG. 16). The lanes in the gel related to the following samples—lane 1=appropriate DNA ladder, lane 2 corresponded to sample 1 (a control showing the band produced by unmodified SEQ ID NO: 29), lane 3 corresponded to sample 2 (the wild-type MuA substrate (MuA substrate 1, SEQ ID NO: 27 attached to SEQ ID NO: 24 by four C3 spacer units, hybridised to SEQ ID NO: 28) which did not contain at least one nucleoside which was not present in the template polynucleotide (SEQ ID NO: 29)), lane 4 corresponded to sample 3 (the MuA substrate which contained a dUMP (MuA substrate 2, SEQ ID NO: 27 attached to SEQ ID NO: 24 by four C3 spacer units, hybridised to SEQ ID NO: 26) which was not present in the template polynucleotide (SEQ ID NO: 29)), lane 5 corresponded to sample 4 (the broad band on the gel indicated that the MuA transposase was able to insert the wild-type MuA substrate (SEQ ID NO: 27 attached to SEQ ID NO: 24 by four C3 spacer units, hybridised to SEQ ID NO: 28) into the template polynucleotide (SEQ ID NO: 29)), lane 6 corresponded to sample 5 (the broad band on the gel indicated that the MuA transposase was able to insert the MuA substrate which contained a dUMP (SEQ ID NO: 27 attached to SEQ ID NO: 24 by four C3 spacer units, hybridised to SEQ ID NO: 26) into the template polynucleotide (SEQ ID NO: 29)). Lanes 3 and 4 produced a faint band at the bottom of the gel as they were run as additional controls. FIG. 16 shows that MuA transposase was able to insert MuA substrates which contained at least one nucleoside which was not present in the template polynucleotide (MuA substrate 2).

TABLE 5 Final Reagent Sample 1 Sample 2 Sample 3 Sample 4 Sample 5 Concentrations Water 5.5 uL 6 uL 6 uL 2.5 uL 2.5 uL Lambda DNA 2.5 uL 2.5 uL 2.5 uL 250 ng (100 ng/uL) MuA substrate 1 2 uL 2 uL 200 nM (1 uM) MuA substrate 2 2 uL 2 uL 200 nM (1 uM) 5x Finnzymes 2 uL 2 uL 2 uL 2 uL 2 uL 1x reaction buffer MuA (4 uM, 1 uL 1 uL 400 nM Thermo, catalogue No. F-750C) Total 10 uL 10 uL 10 uL 10 uL 10 uL

Example 2

This example describes the sample preparation process described in FIG. 17 section A). In step 1 ssDNA fragments were annealed in order to form a model construct (X). In step 2 the dUMP (labelled as a triangle) was removed using USER™. Finally, in step 3 the gap in construct x was repaired using a DNA polymerase and ligase, to produce a modified ds-DNA polynucleotide. FIG. 17 section B shows construct Y the ds DNA control.

Materials and Methods

2.1 Anneal of DNA Strands to Form Construct X and Y

Constructs X and Y were prepared as shown in Table 6 below. The sample mixtures which contained the DNA to form constructs X and Y were then heated to 95° C. for 2 minutes and then cooled to 16° C. at a rate of 2° C. per minute. This allowed SEQ ID NO: 30 to anneal to either SEQ ID NO: 31 and 32 (construct X) or SEQ ID NO: 33 (construct Y, the control). The DNA substrate designs of the two constructs formed are shown in FIGS. 17 A (after Step 1) and B.

TABLE 6 Final Reagent Construct X Construct Y Concentrations Water 0.5 uL 3 uL 0.25M NaCl, 2 uL 2 uL 50 mM, 50 mM Tris pH 7.5 10 mM SEQ ID NO: 30 (100 uM) 2.5 uL 2.5 uL 25 uM SEQ ID NO: 31 (100 uM) 2.5 uL 25 uM SEQ ID NO: 32 (100 uM) 2.5 uL 25 uM SEQ ID NO: 33 (100 uM) 2.5 uL 25 uM Total 10 uL 10 uL 2.2 USER Digest of Construct X

An aliquot of the construct X solution that was annealed in the previous step was then treated with USER™ digest in order to remove the dUMP from SEQ ID NO: 32. See Table 7 below for appropriate volumes and concentrations. The sample was then incubated at 37° C. for 30 minutes before it was cooled in an ice block.

TABLE 7 Final Reagent Construct X Concentrations Water 22.8 uL Construct X (25 uM) 1.2 uL 1 uM (30 pmol) 10x DNA ligase buffer 3 uL 1x USER (1 U/uL) 3 uL 3 U Total 30 uL 2.3 Repair of Single-Stranded Gap in the Double-Stranded DNA Construct

The construct X sample produced after treatment with USER™ was then aliquoted out into three batches of 10 μL. One batch was treated with DNA polymerase only (3 in Table 8), the second was treated with DNA polymerase and ligase (4 in Table 8) and the third was not incubated with either enzyme as a control (2 in Table 8). An additional control experiment (1 in Table 8) was also run where the ds construct Y was incubated in buffer only (in the absence of both polymerase and ligase). Samples 1-4 of Table 8 were incubated for 30 minutes at 16° C. and then EDTA (0.5 M, 5 μL) was added to each sample. A PCR purification kit was then used to purify each sample, which was eluted in 20 μL.

TABLE 8 Final Concen- Reagent 1 2 3 4 trations Water 17.2 uL 7.6 uL 7.1 uL 6.6 uL Construct X sample 10 uL 10 uL 10 uL 0.5 uM from Table 7 (1 uM) Construct Y sample 0.4 uL 0.5 uM from Table 7 (25 uM) 10x DNA ligase 2 uL 2 uL 2 uL 2 uL 1x buffer dNTPs (10 mM) 0.4 uL 0.4 uL 0.4 uL 0.4 uL 200 uM T4 DNAP exo(−) 0.5 uL 0.5 uL Quick Ligase (T4 0.5 uL 1x DNA ligase) Total 20 uL 20 uL 20 uL 20 uL 2.4 Digestion of DNA Samples

Aliquots (8.5 μL) of samples 1-4 in Table 8 were then subjected to either T7 digestion conditions or a control incubation in buffer in the absence of enzyme (see Table 9 below). The samples were incubated at 37° C. for 5 minutes and then EDTA was added to each sample (0.5 M, 5 μL). Finally, all eight samples (a-h) were run on an Agilent 1000 chip (See FIG. 18).

TABLE 9 Final Reagent a b c d e f g h Concentration Water 0.5 uL 0.5 uL 0.5 uL 0.5 uL 1 from 8.5 uL 8.5 uL 0.4 uM Table 8 (0.5 uM) 2 from 8.5 uL 8.5 uL Table 8 (0.5 uM) 3 from 8.5 uL 8.5 uL Table 8 (0.5 uM) 4 from 8.5 uL 8.5 uL Table 8 (0.5 uM) 10x 1 uL 1 uL 1 uL 1 uL 1 uL 1 uL 1 uL 1 uL 1x NEBuffer 4 T7 0.5 uL 0.5 uL 0.5 uL 0.5 uL 1x Total 10 uL 10 uL 10 uL 10 uL 10 uL 10 uL 10 uL 10 uL

FIG. 18 shows that construct Y had not been digested by the T7 enzyme because it was a ds DNA construct with no single-stranded DNA gaps (FIG. 18 lane 3). Construct X was susceptible to digestion when it had not been exposed to either DNA polymerase or DNA ligase (see col. 5 of FIG. 18) or when construct X had only been exposed to DNA polymerase (T7 enzyme) (col. 7 of FIG. 18). When construct X had been treated with both DNA polymerase, to fill in the gap in the upper strand, and then ligase, to join the short fragments of DNA together, it was not susceptible to digestion by T7 enzyme (col. 9 of FIG. 18). Therefore, but treating construct X with USER, then DNA polymerase and ligase, it was possible to fill in the single-stranded DNA gap and form a complementary strand of DNA (SEQ ID NO: 33) to the DNA template (SEQ ID NO: 30).

Example 3

This example describes the sample preparation process described in FIG. 19 which was used to fragment Lambda DNA into ˜5-10 kB fragments. The fragments which were produced by the sample prep were then passed through a nanopore, with their movement controlled by a helicase enzyme. Markers which were present in the Y-shaped MuA substrates and the hairpin MuA substrates (iSpC3 spacers) that were inserted into the double-stranded Lambda DNA (SEQ ID NO: 29 shows the template strand sequence only) were identified by characteristic current blockades. The observance of the characteristic blocks showed that the sample preparation procedure had been successful.

Materials and Methods

3.1 Anneal of DNA Strands to Form Y-Shaped and Hairpin MuA Substrates

The Y-shaped and hairpin MuA substrates were prepared as shown in Table 10 below. The sample mixtures which contained the DNA to form the Y-shaped and hairpin MuA substrates were then heated to 95° C. for 2 minutes and then cooled to 16° C. at a rate of 2° C. per minute. This allowed SEQ ID NOS: 34 and 35 (where SEQ ID NO: 34 attached at its 3′ end to the 5′ end of SEQ ID NO: 35 by four iSpC3 spacer units) to anneal to SEQ ID NOS: 36 and 37 (where SEQ ID NO: 36 attached at its 3′ end to the 5′ end of SEQ ID NO: 37 by four iSpC3 spacer units) to form the Y-shaped MuA substrate and for SEQ ID NOS: 36 and 35 (where SEQ ID NO: 36 attached at its 3′ end to the 5′ end of SEQ ID NO: 35 by four iSpC3 spacer units) to form a hairpin loop MuA substrate. The DNA substrate designs of the two MuA substrates formed are shown in FIG. 20 (a).

TABLE 10 Final Concen- Reagent Y-shaped Hairpin trations Water 12 uL 14 uL 0.5M NaCl 2 uL 2 uL 50 mM 0.1M Tris pH 7.5 2 uL 2 uL 10 mM SEQ ID NO: 34 and 35 (where SEQ ID 2 uL 10 uM NO: 34 attached at its 3′ end to the 5′ end of SEQ ID NO: 35 by four iSpC3 spacer units) (100 uM) SEQ ID NO: 36 and 37 (where SEQ ID 2 uL 10 uM NO: 36 attached at its 3′ end to the 5′ end of SEQ ID NO: 37 by four iSpC3 spacer units) (100 uM) SEQ ID NO: 36 and 35 (where SEQ ID 2 uL 10 uM NO: 36 attached at its 3′ end to the 5′ end of SEQ ID NO: 35 by four iSpC3 spacer units) (100 uM) Total 20 uL 20 uL 3.2 Fragmentation of the DNA Template Using the MuA Transposase

Double-stranded Lambda DNA (SEQ ID NO: 29 shows the template strand only) was fragmented into approximately 5-10 kB length strands using a MuA transposase. The MuA transposase inserted the MuA substrates (the Y-shaped and the hairpin MuA substrates) which were annealed in section 3.1. The sample was prepared as shown in Table 11 below. The sample was then incubated at 30° C. for 1 hour and heat inactivated at 75° C. for 10 minutes. The sample was then further purified using a PCR Purification kit and eluted in 26 μL.

TABLE 11 Sample Final Reagent Volume 1 Concentrations Water 17.7 uL Lambda DNA (90 ng/uL) (SEQ ID NO: 22.3 uL 2 μg 29 shows the template strand sequence only) Y-shaped MuA substrate (1 uM) 8 uL 100 nM Hairpin MuA substrate (1 uM) 8 uL 100 nM 5x Buffer (125 mM Tris (pH 8.0), 16 uL 1x 50 mM MgCl₂, 550 mM NaCl, 0.25% Triton X-100 and 50% Glycerol) MuA (4 uM, Thermo, catalogue No. F- 8 uL 400 nM 750C) Total 80 uL 3.3 USER Digest of Fragmented Lambda DNA with Inserted MuA Substrates

Purified sample volume 1 from step 3.2 was then treated with USER™ digest in order to remove the dUMP from SEQ ID NO: 36. See Table 12 below for appropriate volumes and concentrations. The sample was then incubated at 37° C. for 30 minutes before it was cooled in an ice block.

TABLE 12 Final Reagent Sample Volume 2 Concentrations Sample Volume 1 26 uL 8 pmol of U 10x DNA ligase buffer 3 uL 1x USER (1 U/uL) 1 uL 1 U Total 30 uL 3.4 Repair of Single-Stranded Gap in the Double-Stranded Lambda DNA Construct Fragments

Sample Volume 2 produced after treatment with USER™ was then treated with DNA polymerase and ligase in order to close the single-stranded gap. Sample volume 3 (see table 13 below for appropriate volumes and concentrations) was incubated for 30 minutes at 16° C. and then EDTA (0.5 M, 10 μL) was added to sample volume 3. A PCR Purification kit was then used to purify each sample, which was eluted in 50 μL of water. An aliquot of the purified sample (1 μL) was run on an Agilent 12000 chip to quantify the sample and Tris-HCl and NaCl (pH 7.5) were added to the rest of the sample until the concentrations were 10 mM and 50 mM respectively. Finally, SEQ ID NO: 39 (3′ end of the sequence had six iSp18 spacers attached to two thymine residues and a 3′ cholesterol TEG, 0.5 μM) was annealed to the purified sample.

TABLE 13 Final Reagent Sample Volume 3 Concentrations Water 6.2 uL Sample Volume 2 30 uL 10x DNA ligase buffer 1 uL 1x dNTPs (10 mM) 0.8 uL 200 uM T4 DNAP exo(−) (Lucigen) 1 uL Ligase (NEB; M0202M) 1 uL 1x Total 40 uL 3.5 Electrophysiology Experiment Showing Helicase Controlled DNA Movement of the Purified and Fragmented Lambda DNA Construct

Prior to setting up the experiment, the Lambda DNA construct (0.2 nM, 5-10 kB fragments of Lambda DNA which had the Y-shaped MuA substrates and the hairpin MuA substrates attached to either end of the fragments by the MuA transposase (see FIG. 20 (b) for an example construct)) and Trwc Cba (SEQ ID NO: 40, 1 μM) were pre-incubated together for 1 hour in buffer (50 mM CAPS, pH 10.0 (pH altered to pH 10.0 by addition of NaOH), 100 mM NaCl).

Electrical measurements were acquired from single MspA nanopores (MS(B1-G75S/G77S/L88N/Q126R)8 MspA (SEQ ID NO: 2 with mutations G75S/G77S/L88N/Q126R) inserted in block co-polymer in buffer (600 mM KCl, 25 mM KH2PO4, 75 mM Potassium Ferrocyanide (II), 25 mM Potassium ferricyanide (III), pH 8). After a single pore in the bilayer was achieved, then buffer (1 mL, 600 mM KCl, 25 mM KH2PO4, 75 mM Potassium Ferrocyanide (II), 25 mM Potassium ferricyanide (III), pH 8) was flowed through the system to remove any excess MspA nanopores (MS(B1-G75S/G77S/L88N/Q126R)8 MspA (SEQ ID NO: 2 with mutations G75S/G77S/L88N/Q126R) and the experimental system was placed on a cooler plate set to 8° C. which gave a system temperature of ˜15° C. MgCl₂ (10 mM) and dTTP (5 mM) were mixed together with buffer (600 mM KCl, 25 mM KH2PO4, 75 mM Potassium Ferrocyanide (II), 25 mM Potassium ferricyanide (III), pH 8) and then added to the Lambda DNA construct (0.2 nM), Trwc Cba (SEQ ID NO: 40, 1 μM) buffer (50 mM CAPS, pH 10.0 (pH altered to pH 10.0 by addition of NaOH), 100 mM NaCl) pre-mix. The pre-mix was then added to the single nanopore experimental system. Experiments were carried out for two hours following a potential flip process (+120 mV for 30 mins, then −100 mV for 2 seconds and then 0 mV for 2 seconds) and helicase-controlled DNA movement was monitored.

3.6 Results and Discussion

Helicase controlled DNA movement was observed for the Lambda DNA construct, an example of a helicase-controlled DNA movement is shown in FIG. 21. The iSpC3 spacers present in the Lambda DNA construct produced a characteristic block level highlighted by the numbers 1, 2 and 3 in FIG. 21. The Y-shaped MuA substrate had four iSpC3 spacers in either strand and the hairpin MuA substrate also had four iSpC3 spacers, each iSpC3 spacer allowed more current to flow as that region of the Lambda DNA construct translocated through the nanopore. If the sample preparation had occurred successfully then the iSpC3 spacer events would have been observed at the beginning of the Lambda DNA construct, in the middle (marking the transition between template and template complement sequences) and at the end. FIG. 21 clearly shows three instances of increased current flow which corresponded to the iSpC3 spacer regions. Therefore, the sample preparation procedure effectively introduced MuA substrates into the Lambda DNA and produced the Lambda DNA constructs shown in FIG. 20 (b).

Example 4

This example describes an alternative sample preparation process illustrated in FIG. 22 which was used to fragment Lambda DNA into ˜5-10 kB fragments. The fragments which were produced by the sample prep were then passed through a nanopore, with their movement controlled by a helicase enzyme. Markers which were present in the Y-shaped MuA substrates and the hairpin MuA substrates (iSpC3 spacers) that were inserted into the double-stranded Lambda DNA (SEQ ID NO: 29 shows the template strand sequence only) were identified by characteristic current blockades. The observance of the characteristic blockades showed that the sample preparation procedure had been successful.

Materials and Methods

4.1 Anneal of DNA Strands to Form Y-Shaped and Hairpin MuA Substrates

The Y-shaped and hairpin MuA substrates were prepared as described in Example 3.1 above. Volumes, concentrations and sequences that were used in this example are detailed in table 14 below. The DNA substrate designs of the two constructs formed are shown in FIG. 23 (a).

TABLE 14 Final Y-shaped Hairpin Concen- Reagent 2 2 trations Water 12 uL 14 uL 0.5M NaCl 2 uL 2 uL 50 mM 0.1M Tris pH 7.5 2 uL 2 uL 10 mM SEQ ID NO: 34 and 35 (where SEQ ID 2 uL 10 uM NO: 34 is attached at its 3′ end to the 5′ end of SEQ ID NO: 35 by four iSpC3 spacer units) (100 uM) SEQ ID NO: 41 and 37 (where SEQ ID 2 uL 10 uM NO: 41 is attached at its 3′ end to the 5′ end of SEQ ID NO: 37 by four iSpC3 spacer units) (100 uM) SEQ ID NO: 41 and 35 (where SEQ ID 2 uL 10 uM NO: 41 is attached at its 3′ end to the 5′ end of SEQ ID NO: 35 by four iSpC3 pacer units) (100 uM) Total 20 uL 20 uL 4.2 Fragmentation of the DNA Template Using the MuA Transposase

Double-stranded Lambda DNA (SEQ ID NO: 29) was fragmented into approximately 5-10 kB length strands using a MuA transposase. The MuA transposase inserted the MuA substrates (the Y-shaped 2 and the hairpin 2 MuA substrates) which were annealed in section 4.1. The sample was prepared by an analogous procedure as that described in Section 3.2 and table 11 above except the MuA substrates used were the Y-shaped 2 and the hairpin 2 MuA substrates. In this case the purified sample X was eluted in a volume of 20 μL.

4.3 Nick Repair in the Double-Stranded Lambda DNA Construct Fragments

Once the Y-shaped 2 and the hairpin 2 MuA substrates had been inserted into the fragmented Lambda DNA it was necessary to repair the nick in the strand and join the inosines to the Lambda DNA fragment to produce a complete double-stranded Lambda DNA fragment. The reaction was assembled on ice as described in Table 15 below. The sample was incubated at 16° C. for 60 mins before EDTA (10 μL, 0.5 M) was added to the sample. The resultant sample mixture was purified and was eluted in 50 μL of water. An aliquot of the purified sample (1 μL) was run on an Agilent 12000 chip to quantify the sample and Tris-HCl and NaCl (pH 7.5) were added to the rest of the sample until the concentrations were 10 mM and 50 mM respectively. Finally, SEQ ID NO: 39 (3′ end of the sequence had six iSp18 spacers attached to two thymine residues and a 3′ cholesterol TEG, 0.5 μM) was annealed to the purified Lambda DNA construct.

TABLE 15 Final Reagent Sample Z Concentrations Sample X 16 uL 2x DNA ligase buffer 20 uL 1x Ligase (NEB; M0202M) 4 uL Total 40 uL 4.4 Electrophysiology Experiment Showing Helicase Controlled DNA Movement of the Purified and Fragmented Lambda DNA Construct

Prior to setting up the experiment, the Lambda DNA construct (0.2 nM, 5-10 kB fragments of Lambda DNA which had the Y-shaped 2 and the hairpin 2 MuA substrates attached to either end of the fragments by the MuA transposase (see FIG. 23 (b) for an example construct)) and Trwc Cba (SEQ ID NO: 40, 1 μM) were pre-incubated together for 1 hour in buffer (50 mM CAPS, pH 10.0 (pH altered to pH 10.0 by addition of NaOH), 100 mM NaCl).

Electrical measurements were acquired from single MspA nanopores (MS(B1-G75S/G77S/L88N/Q126R)8 MspA (SEQ ID NO: 2 with mutations G75S/G77S/L88N/Q126R) inserted in block co-polymer in buffer (600 mM KCl, 25 mM KH2PO4, 75 mM Potassium Ferrocyanide (II), 25 mM Potassium ferricyanide (III), pH 8). After achieving a single pore in the bilayer, then buffer (3 mL, 600 mM KCl, 25 mM KH2PO4, 75 mM Potassium Ferrocyanide (II), 25 mM Potassium ferricyanide (III), pH 8) was flowed through the system to remove any excess MspA nanopores (MS(B1-G75S/G77S/L88N/Q126R)8 MspA (SEQ ID NO: 2 with mutations G75S/G77S/L88N/Q126R) and the experimental system was placed on a cooler plate set to 8° C. which gave a system temperature of −15° C. MgCl₂ (10 mM) and dTTP (5 mM) were mixed together with buffer (600 mM KCl, 25 mM KH2PO4, 75 mM Potassium Ferrocyanide (II), 25 mM Potassium ferricyanide (III), pH 8) and then added to the Lambda DNA construct (0.2 nM), Trwc Cba (SEQ ID NO: 40, 1 μM) buffer (50 mM CAPS, pH 10.0 (pH altered to pH 10.0 by addition of NaOH), 100 mM NaCl) pre-mix. The pre-mix was then added to the single nanopore experimental system. Experiments were carried out for two hours following a potential flip process (+120 mV for 30 mins, then −100 mV for 2 seconds and then 0 mV for 2 seconds) and helicase-controlled DNA movement was monitored.

4.5 Results and Discussion

Helicase controlled DNA movement was observed for the Lambda DNA construct, an example of a helicase-controlled DNA movement is shown in FIG. 24. The iSpC3 spacers present in the Lambda DNA construct produced a characteristic block level highlighted by numbers 1-3 in FIG. 24. The Y-shaped 2 MuA substrate had four iSpC3 spacers in either strand and the hairpin 2 MuA substrate also had four iSpC3 spacers, each iSpC3 spacer allowed more current to flow as that region of the Lambda DNA construct translocated through the nanopore. If the sample preparation had occurred successfully then the iSpC3 spacer events would have been observed at the beginning of the Lambda DNA construct, in the middle (making the transition between the template and template complement strands) and at the end. FIG. 24 clearly shows three instances of increased current flow which corresponded to the iSpC3 spacer regions. Therefore, the sample preparation procedure effectively introduced MuA substrates into the Lambda DNA and produced the Lambda DNA constructs shown in FIG. 23 (b). Owing to the use of the inosines in the Y-shaped 2 and the hairpin 2 MuA substrates, the steps in the sample preparation procedure were reduced as once the MuA substrates had been inserted all that was necessary was to close the nicks in the double-stranded DNA constructs. 

The invention claimed is:
 1. A method for modifying a template double stranded polynucleotide, comprising: (a) contacting the template polynucleotide with a MuA transposase and a population of double stranded MuA substrates each comprising (i) at least one overhang and (ii) at least one nucleotide in the same strand as the at least one overhang which comprises a nucleoside that is not present in the template polynucleotide such that the transposase fragments the template polynucleotide and ligates a substrate to one or both ends of the double stranded fragments and thereby producing a plurality of fragment/substrate constructs; (b) removing the overhangs from the constructs by selectively removing the at least one nucleotide and thereby producing a plurality of double stranded constructs comprising single stranded gaps; and (c) repairing the single stranded gaps in the constructs and thereby producing a plurality of modified double stranded polynucleotides.
 2. A method according to claim 1, wherein the template polynucleotide comprises the nucleosides deoxyadenosine (dA), deoxyuridine (dU) and/or thymidine (dT), deoxyguanosine (dG) and deoxycytidine (dC).
 3. A method according to claim 1, wherein the nucleoside that is not present in the template polynucleotide is abasic, adenosine (A), uridine (U), 5-methyluridine (m⁵U), cytidine (C) or guanosine (G) or comprises urea, 5, 6 dihydroxythymine, thymine glycol, 5-hydroxy-5 methylhydanton, uracil glycol, 6-hydroxy-5, 6-dihdrothimine, methyltartronylurea, 7, 8-dihydro-8-oxoguanine (8-oxoguanine), 8-oxoadenine, fapy-guanine, methy-fapy-guanine, fapy-adenine, aflatoxin B1-fapy-guanine, 5-hydroxy-cytosine, 5-hydroxy-uracil, 3-methyladenine, 7-methylguanine, 1,N6-ethenoadenine, hypoxanthine, 5-hydroxyuracil, 5-hydroxymethyluracil, 5-formyluracil or a cis-syn-cyclobutane pyrimidine dimer.
 4. A method according to claim 1, wherein the at least one nucleotide is 10 nucleotides or fewer from the overhang.
 5. A method according to claim 1, wherein the at least one nucleotide is the first nucleotide in the overhang.
 6. A method according to claim 1, wherein all of the nucleotides in the overhang comprise a nucleoside that is not present in the template polynucleotide.
 7. A method according to claim 1, wherein each substrate comprises an overhang at one end and a hairpin loop at the other end and wherein the two strands of the modified double stranded polynucleotides produced by the method are linked by the hairpin loop at one or both ends.
 8. A method according to claim 1, wherein each substrate is a Y substrate with an overhang at one end and a region that is not complementary at the other end and wherein the modified double stranded polynucleotides produced by the method have the non-complementary region at one or both ends.
 9. A method according to claim 1, wherein a proportion of the substrates in the population comprise an overhang at one end and a hairpin loop at the other end and a proportion of the substrates in the population are Y substrates with an overhang at one end and a region that is not complementary at the other end and wherein at least some of the modified double stranded polynucleotides produced by the method have the hairpin loop at one end and the non-complementary region at the other end.
 10. A method according to claim 1, wherein the substrate comprises a selectable binding moiety.
 11. A method according to claim 7, wherein the method further comprises separating the two strands of at least one modified double stranded polynucleotide to produce at least one single stranded polynucleotide comprising one strand of the modified double stranded polynucleotide linked to the other strand of the modified double stranded polynucleotide.
 12. A method of characterising at least one polynucleotide modified using a method according to claim 1, comprising: a) contacting the modified polynucleotide with a transmembrane pore such that at least one strand of the polynucleotide moves through the pore; and b) taking one or more measurements as the at least one strand moves with respect to the pore wherein the measurements are indicative of one or more characteristics of the at least one strand and thereby characterising the modified polynucleotide.
 13. A method of characterising a template polynucleotide, comprising: a) modifying the template polynucleotide using a method according to claim 1 to produce a plurality of modified polynucleotides; b) contacting each modified polynucleotide with a transmembrane pore such that at least one strand of each polynucleotide moves through the pore; and c) taking one or more measurements as the at least one strand of each polynucleotide moves with respect to the pore wherein the measurements are indicative of one or more characteristics of the at least one strand of each polynucleotide and thereby characterising the template polynucleotide.
 14. A method according to claim 13, wherein the one or more characteristics are selected from (i) the length of the polynucleotide, (ii) the identity of the polynucleotide, (iii) the sequence of the polynucleotide, (iv) the secondary structure of the polynucleotide and (v) whether or not the polynucleotide is modified.
 15. A method according to claim 13, wherein step (b) further comprises contacting each modified polynucleotide with a polynucleotide binding protein such that the protein controls the movement of the at least one strand of each polynucleotide through the pore.
 16. A method according to claim 15, wherein the method comprises (a) contacting each modified polynucleotide with a transmembrane pore and a polynucleotide binding protein such that at least one strand of each polynucleotide moves through the pore and the protein controls the movement of the at least one strand of each polynucleotide through the pore; and (b) measuring the current passing through the pore as the at least one strand of each polynucleotide moves with respect to the pore wherein the current is indicative of one or more characteristics of the at least one strand of each polynucleotide and thereby characterising the template polynucleotide.
 17. A method according to claim 15, wherein the polynucleotide binding protein is derived from a helicase. 